On Wednesday, an international team of scientists said they have found the most convincing evidence yet that dark matter exists.
These results are not conclusive — more testing and analysis needs to be done — but the precision of the data is unmatched by any previous experiment, MIT astrophysicist Samuel Ting said in a news conference.
So while we still may not have directly found dark matter, we are closer than ever before.
Here’s where we’ve been since it was first theorized 80 years ago.
Dark matter makes up nearly a quarter of our universe, and yet, so far, scientists have been unable to visually detect dark matter because it is just that -- dark. The mysterious substance does not emit or absorb light, or other forms of electromagnetic waves.
The reason we know dark matter exists is because stars, galaxies, and clusters of galaxies have more gravity, and therefore more mass than would be inferred from their visible appearance in a telescope. To rectify this disparity, researchers have concluded there has to be some missing mass that is invisible to our detectors.
Mass is measured by its gravitational effects on celestial bodies. The more massive something is, the stronger its gravitational pull. Gravity is the glue that holds our solar system, galaxies, and clusters of galaxies together.
Scientists know the mass of visible matter, and that this mass would not create a strong enough gravitational attraction to keep stars and galaxies together. Thus, without the gravitational interactions created by some invisible mass, which we have named 'dark matter,' galaxies would fly apart as they whip around.
Fritz Zwickyin, from the California Institute of Technology, was the first astronomer to notice the discrepancy between the mass of visible matter and the calculated mass of stars and galaxies.
While observing a cluster of galaxies called the Cosizema cluster, Zwickyin calculated it had 400 times more mass than it should have had, based on what he saw with a telescope.
Zwickyin also noticed that motion of the galaxies in the clusters was much too fast to be caused by only visible matter.
The stars and galaxies would fly apart if there weren't some extra mass creating a gravitational attraction that kept them together.
There must be something else there, he theorized. He called this missing mass 'invisible matter' or 'dark matter.'
Vera Rubin saw that bodies at the furthest edges of galaxies didn't move slowly than those at the centre, as Newton's laws said they should.
She theorized that there must be something in the outskirts of galaxies that was causing these objects to move faster than they should. It had to be dark matter.
In the 1970s, Professors James Peebles and Jeremiah Ostriker created computer simulations suggesting there was more mass in the universe than accounted for.
By 1973, Jerry Ostriker and Jim Peebles, both of Princeton University, decided to combine their different skills to build computer models of the universe. But every time they tried to replicate the Milky Way, using what scientists knew about our home galaxy, they failed.
The computer would spit out pictures of blobs or random shapes. Something crucial was missing.
The professors knew that if they added more matter -- a LOT more matter -- they could generate enough gravity to hold their model together.
They knew about Fritz Zwickyin's observations 40 years earlier, so Ostriker and Peebles sat down at their computer and added dark matter to their model. It worked.
Scientists use many ways to try to detect dark matter, but so far, every experiment has yielded inconclusive results.
In 2008, the PAMELA satellite, operated by Russia, Italy, Germany, and Sweden, detected an excess of positrons, the antimatter counterpart of electrons. The extra positrons could be the result of two dark matter particles colliding and decaying, which would be indirect evidence that dark matter exists.
If the positrons were originating from a larger dark matter particle, then scientists would also expect the number of positrons to drop off at an energy level exceeding the maximum possible mass of dark matter particles. Researchers didn't see that.
The first finding from a particle physics detector on the outside of the International Space Station are the most convincing yet.
For the last two years, scientists have been collecting information about cosmic rays -- charged particles flying around our universe -- from an instrument on the International Space Station called the Alpha Magnetic Spectrometer.
The detector records the kinds of particles that pass through it, as well as their mass, speed, and direction of travel.
On Wednesday, scientists announced that they found the same excess of positrons as PAMELA had found a few years earlier. The difference is that this experiment is more precise than any previous experiment.
'AMS is the first experiment to measure to 1 per cent accuracy in space,' Nobel laureate Samuel Ting said in a statement released by CERN. 'It is this level of precision that will allow us to tell whether our current positron observation has a Dark Matter or pulsar origin.'
The positrons could be the result of dark matter particles colliding and destroying each other.
There could also be a more mundane answer. They might just come from pulsars, collapsed stars that spew out charged particles as they rotate.
The good news is that scientists are more determined than ever to find the source of this antimatter signal. The AMS will continue to record data on the International Space Station, meaning every day we get closer and closer to an answer.
'What we have shown today only represents less than 10 per cent of the data,' Ting said. With enough time, 'there's no question we are going to solve this problem' he added.
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