Here's something that blows my mind: we can only see about 5% of the universe! The rest is made up of dark matter (about 27%) and dark energy (about 68%). We have no idea what dark matter actually is!
How Do We Know It's There?
We know dark matter exists because of its gravitational effects on visible matter. Galaxies spin faster than they should based on their visible mass. Something invisible must be adding gravity. That's dark matter.