Dark Matter

 

 Universe is made up of matter. Matter has mass. We calculated the amount of mass in the universe, turns out all of the matter accounts for only about 15% of it. What mass is the rest 85%, that's the mystery.

Dark matter is the hypothetical matter in the remaining 85% ; called "dark" because it doesn't interact with light or any EM wave. It only interacts with gravity, which is how we know it exists. Scientists observed that the gravitational effects in galaxies and galaxy clusters couldn't really be explained by the amount of visible matter. Galaxies would not be held together if it weren't for another entity acting as an adhesive, that's where dark matter comes in.

We have no idea what dark matter is made up of. There are a few theories about "WIMPS" (Weakly interacting massive particles) or axions which are fluid-like structures, but it's still a big mystery.