See? Complex. Dark matter is similar to what we think of as normal matter — the kind we're made of — in that is has mass and therefore gravity. But it doesn't emit light and doesn't interact with normal matter, making it the devil's own game to detect. We have plenty of reasons to think it exists, but we've never been able to directly detect it. Video of Dark Matter: Crash Course Astronomy #41 A lot of potential dark matter candidates have been eliminated over the years, leaving only ever more exotic possibilities, like subatomic particles that have not yet been discovered. The leading ca...