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The universe has many more unknowns than knowns. But one of it’s biggest mysteries is called ‘dark matter’.

When we look at our Milky Way galaxy, or any galaxy in the universe, we see things like stars, black holes, and planets. They all have mass and when we add them up, we should end up with the total mass of the galaxy. Or so we thought.

When we put this number into a simulation, galaxiesΒ fly apart instead of rotating. The total mass is not enough to hold the galaxy together. To get the simulations to reflect what we see through our telescopes, the mass of each galaxy needs to be higher.

The best solution we have to this problem is that, spread throughout the galaxy, there must be a ghostly substance that takes up the rest of the missing mass. It must be ghostly because we can’t yet see or detect it in any way. This is why we’ve named it ‘dark matter’.

It has to be made up of some subatomic particle that we haven’t found yet, and that doesn’t interact with light or normal matter – just gravity.

When we adjust our simulations to include dark matter, suddenly the math works out. The galaxy sticks together.

But for the simulations to work, each galaxy has to have way moreΒ dark matter than ordinary matter – by almost 8 to 1.

Other than these pretty scarce predictions, we don’t know a single thing about dark matter. But because there’s so much of it, it means that we don’t understand the substance that makes upΒ 85% of all matter in the universe.

Just like with dark energy, its exact nature remains a major current mystery in physics.

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Ben McCarthy

Ben McCarthy

Ben is the Founder of Discover Earth and the author of the Big Ideas Network.