If an alien managed to visit our Universe from a parallel reality, there is a high chance they would not even notice we exist. In a way that's obvious: the Universe is enormous and our planet is but a small, pale blue dot. But it's worse than that: the aliens might not even notice all the stars and the planets that orbit them. They could even miss the vast clouds of dust that float through space.
All these familiar things only make up a fraction of the matter in our Universe. The rest is something else, a material that nobody on Earth has ever seen.
For want of a better name, physicists call this stuff "dark matter". If it weren't there, galaxies would fly apart. Nobody knows what it is, but physicists are hot on its trail.
Everything you see around you, from your own body to the planet you're standing on and the stars in the sky, is made of atoms. These in turn are made up of smaller particles like protons and neutrons, many of which can be broken down even further.
When physicists began to understand the makeup of atoms in the early 20th century, it seemed that we were about to understand the basis of all the matter in the Universe.
But in 1933, a Swiss astronomer called Fritz Zwicky began arguing that most of the Universe had to be made of something else entirely.
Zwicky counted up all the material he could observe in groups of galaxies. He found that there was not enough matter there to account for the force of gravity holding them together.
This was a crazy theorist who couldn't get his forces to add up
The galaxies that Zwicky observed were also spinning so fast that they should have flung themselves off and scattered into all corners of the Universe, says Richard Massey of Durham University in the UK. Each galaxy was like a merry-go-round that is spinning too fast: any riders would be thrown off.
Zwicky realised there must be something else there, which he could not directly observe, but that had a strong enough gravitational pull to hold everything together. He said that this unknown form of matter was "dark".
At the time he was considered an eccentric and his theories were not taken seriously. "This was a crazy theorist who couldn't get his forces to add up, and so invented an entire new form of matter," says Massey.
Zwicky's work was largely forgotten until the 1970s, when astronomer Vera Rubin discovered that nearby galaxies were not spinning in the right way.
Something must have been there to keep these stars from flying away
In our solar system, a simple rule applies. The further a planet is from the Sun, the weaker gravity's hold is. As a result this planet will move slower, and take longer to complete an orbit.
The same logic should apply to stars orbiting the centre of a galaxy. The stars furthest away should move slowest as the grip of gravity weakens.
Instead, Rubin found that the stars furthest out move just as quickly as nearby stars.
Something must have been there to keep these stars from flying away. Zwicky had been on the right track after all.
Astronomers now believe that dark matter has been fundamental in creating the Universe as we know it.
Dark matter is like the wind: we can't directly see it, but we know it's there
Almost 14 billion years ago, moments after the Big Bang, the Universe began expanding rapidly and clusters of galaxies started forming.
However, the Universe did not expand so fast that all these galaxies flew away into far flung corners. That's because dark matter anchors everything together, despite being invisible.
In one sense dark matter is like the wind: we can't directly see it, but we know it's there. What's more, there is a lot of it: about 25% of the Universe.
Confusingly, it's sometimes said that dark matter makes up about 80% of all the matter in the Universe. That's because only 30% of the Universe is made up of matter, and most of it is dark matter. The rest is energy.
By the 1980s, the first solid evidence for dark matter was coming through.
Dark matter is the skeleton on which ordinary matter hangs
For example, in 1981 a team led by Marc Davis of Harvard University performed one of the first galactic surveys. They realised that galaxies were not arranged in a uniform patterns. They are "not just sprinkled around like icing on a cake", says Carlos Frenk of the University of Durham in the UK.
Instead galaxies congregate into big clusters, each containing hundreds of thousands of galaxies. These make intricate patterns known as the "cosmic web". This web is tied together with dark matter.
In other words, dark matter is the skeleton on which ordinary matter hangs, says Carolin Crawford of the University of Cambridge in the UK. "We know it needed to be around in the early Universe. It's crucial to get that stuff clustered together that will then go on to develop the structures we see."
The discovery of these clusters caused a sensation, says Frenk. Davis, his boss at the time, challenged him to figure out why galaxies were arranged this way.
When Frenk started his search, he discovered that someone claimed to have beaten him to it. In 1980 a Russian team led by VA Lyubimov had set out a possible explanation of dark matter. They proposed that it was made of neutrinos.
We found a Universe with hot dark matter didn't look anything like a real Universe
It made a certain amount of sense. Neutrinos are dark, ghostly particles that barely interact with anything else. The researchers suggested that the combined mass of all the neutrinos in the Universe might account for the missing mass.
There was one issue. Neutrinos are "hot dark matter", meaning they are light and therefore able to move fast. When Frenk simulated a cosmos full of hot dark matter, he found it could not work.
"To our great disappointment we found a Universe with hot dark matter didn't look anything like a real Universe," says Frenk. "It was pretty but not one in which we live. There was this enormous supercluster of galaxies, which we knew did not exist."
Instead, dark matter must be cold and slow-moving. The next step was to find out where this cold dark matter is.
Although we can't see it directly, dark matter does do one thing to give itself away. It bends the light that passes through it. It's a bit like when light shines through a swimming pool or a frosted bathroom window.
We at least have a rough idea of where the dark matter is
The effect is called "gravitational lensing" and it can be used to figure out where the clouds of dark matter are. Using this technique, scientists are creating maps of the Universe's dark matter.
At the moment they have only mapped a fraction. But the team behind one such project has ambitious aims, hoping to map one-eighth of our Universe, amounting to millions of galaxies. To put that in context, our own galaxy, the Milky Way, contains billions of stars and possibly as many as 100 billion planets.
For now these maps are too crude to show any detail. It's like saying you have a basic idea of the continents on Earth but what you're really interested in is the shapes of the mountains and lakes, says Gary Prezeau at Nasa's Jet Propulsion Laboratory at the California Institute of Technology.
Still, we at least have a rough idea of where the dark matter is. But we still don't know what it is.
Several ideas have been put forward, but right now the most popular suggestion is that dark matter is made of a new kind of particle, predicted by theory but never detected. They are called WIMPs: Weakly Interacting Massive Particles.
The term "WIMP" is just a catchphrase, and could include many different types of particles
WIMPs are weak in every sense of the world, says Anne Green of the University of Nottingham in the UK. First, they barely interact with each other, let alone normal matter. When you hit a wall, your hand collides with it, but when a WIMP hits a wall or itself, it will usually pass straight through.
The second part of the acronym speaks for itself. WIMPs have a lot of mass, although they are not necessarily large. They could weigh hundreds or thousands of times more than a proton, says Green.
The thing is, we don't know.
The term "WIMP" is just a catchphrase, and could include many different types of particles, says Massey. Worse, because they are supposedly so ghostly, they are extremely difficult to detect.
At this point you may be throwing your arms up in frustration. "First they decided there's all this invisible matter, now they've decided it's made of some new kind of stuff that they can't detect! This is silly." Well, you're not the first person to say it.
As far back as 1983, some physicists have been arguing that dark matter doesn't exist at all. Instead, the laws of gravity as we know them must be wrong, and that's why galaxies behave so oddly. This idea is called MOND, short for "Modified Newtonian Dynamics".
Anyone who wants to invent a new theory of gravity has to go one better than Einstein
"We're interpreting all of these merry-go-rounds of the Universe, how they are whizzing around and being pulled around by gravity, assuming that we know how gravity works," says Massey. "Maybe we got gravity wrong and are misinterpreting the evidence."
The problem, says Massey, is that the MOND supporters have not come up with a viable alternative to dark matter: their ideas can't explain the data. "Anyone who wants to invent a new theory of gravity has to go one better than Einstein and explain everything he was able to explain, and also account for the dark matter."
In 2006, NASA put out a spectacular image that, for many researchers, killed off MOND for good.
The image shows two enormous clusters of galaxies colliding. As most of the matter is clearly visible in the centre, this is where you would expect most of the gravity to exist.
There are three different ways to find dark matter
But the outer regions show light that is also being bent by gravity, implying that there is another form of matter in those areas. The image was hailed as direct proof of the existence of dark matter.
If that's right, we're back where we were. The challenge is to find dark matter when we don't know what we're looking for.
It may sound worse than the old needle-in-a-haystack problem, but in fact there are three different ways to find it.
The first way is to observe dark matter in action in the cosmos. By monitoring how it behaves using the existing dark matter "maps", astronomers may be able to detect an occasional crash.
They found an area of our Milky Way galaxy that seems to be glowing with gamma rays
Dark matter particles usually pass through normal matter. But the sheer number of them means that, very occasionally, some will collide with the nucleus of an atom.
When this happens, the dark matter "kicks" the atom, making it recoil like a pool ball. This collision should create gamma rays: extremely high-energy light. On these rare occasions, "dark matter can shine," says Frenk.
"There are direct detection experiments which are looking for these nuclear recoils," says Green.
In 2014, using data from NASA's powerful Fermi telescope, researchers claimed to have detected the gamma rays from these collisions. They found an area of our Milky Way galaxy that seems to be glowing with gamma rays, possibly from dark matter.
The patterns fit theoretical models, but the jury is still out on whether the gamma rays are really from dark matter. They could also have come from energetic stars called pulsars, or from collapsing stars.
As well as colliding with normal matter, dark matter might occasionally bump into itself, and there's a way to see that too.
You cannot grab a galaxy-sized cloud of dark matter and put it under a microscope
Massey's team has recently monitored galaxies smashing into each other. They expected all the dark matter in the galaxies to pass straight through, but instead some of it slowed down, lagging behind the galaxy it belonged to.
This indicates it had interacted with other dark matter. "If it did, then that's the first evidence that it cares just a tiny bit about the rest of the world," says Massey.
Both these methods have a major drawback: you cannot grab a galaxy-sized cloud of dark matter and put it under a microscope. They're too big and too far away.
So a second way of detecting dark matter would be to create it first.
Physicists hope to do just that using particle colliders, like the Large Hadron Collider (LHC) in Geneva, Switzerland.
The LHC smashes protons together at speeds close to that of light. These collisions are powerful enough to break the protons down into their constituent parts. The LHC then studies this subatomic debris.
During these powerful collisions, new particles such as WIMPs could well be discovered, says Malcolm Fairbairn of Kings College London in the UK.
"If WIMPs do make up the dark matter and we discover them at the LHC then we are in with a good chance of working out what the dark matter in the Universe is composed of," he says.
However, if dark matter is not like a WIMP, the LHC will not detect it.
Scientists are waiting for the rare occasions when WIMPs collide with normal matter
There's another difficulty. If the LHC does create some dark matter, it would not actually register on its detectors.
Instead, the system might find a group of particles moving in one direction but nothing in the other, says Fairbairn. The only way that could happen is if there was something else on the move that the detectors could not pick up. "That might then be a dark matter particle."
If this also fails, the physicists have a third option to fall back on: travel deep underground.
In old mines and inside mountains, scientists are waiting for the rare occasions when WIMPs collide with normal matter – the same sort of collisions the Fermi telescope may have observed in deep space.
Billions of dark matter particles pass through us every second. "They are in your office, in your room, everywhere," says Frenk. "They are crossing through your bodies at a rate of billions per second and you feel nothing."
There have been some false alarms along the way
In theory we should be able to spot the little flashes of gamma rays from these collisions. The trouble is, lots of other things are also passing through, including radiation in the form of cosmic rays, and this swamps the signal from the dark matter.
Hence the underground experiments: the rocks above block most radiation, but allow dark matter through.
So far, most physicists agree we have not yet seen any convincing signals from these detectors. A paper published in August 2015 explains that the XENON100 detector in Italy's Gran Sasso National Laboratory has failed to find anything.
There have been some false alarms along the way. Another team from the same laboratory, using a different detector, have claimed for years that their DAMA experiment had detected dark matter. They do seem to have found something, but most physicists say it is not a WIMP.
One of these detectors, or the LHC, may yet find some dark matter. But finding it in one place won't be enough.
It's a humbling reminder of how far we still have to go before we really understand our Universe
"Ultimately we will have to discover dark matter in more than one way to be sure that the thing we are observing in the laboratory is the same stuff flying round in galaxies," says Fairbairn.
For now, most of our Universe remains dark, and it's not clear how long it will stay that way.
Some cosmologists, Frenk among them, are hopeful that we will get some answers in the next decade. Others, like Green, are less confident. If the LHC doesn't find something soon, she says, we're probably looking for the wrong thing.
It's been over 80 years since Zwicky first suggested the existence of dark matter. In all that time, we haven't been able to get hold of a sample, or nail down what it is.
It's a humbling reminder of how far we still have to go before we really understand our Universe. We may understand all sorts of things, from the beginning of the Universe to the evolution of life on Earth. But most of our Universe is still a black box, its secrets waiting to be unlocked.