Scientists have moved a big step closer to answering one of the greatest mysteries in the universe - what's it made up of?
Even though there are lots of planets and stars, most of the universe is actually invisible.
This mysterious missing stuff is called dark matter.
Now scientists have worked out how fast it moves and how it's spread across the universe by looking at the way stars and galaxies move.
Normal matter which makes up stars, planets, people, animals - in fact everything you see - only accounts for about four per cent of the stuff in the universe.
Scientists know that dark matter is there because it affects everything in space. If there wasn't dark matter stars would fly off in all directions!
The new work will help them find out what this mysterious matter is.