WARNING: READ AT YOUR OWN RISK!
It was, I believe, in 1933 when Swiss astronomer Fritz Zwicky, while observing the orbital velocities of galaxies in galactic clusters, first noticed a mathematical anomaly within their movement. They were not moving in accordance with the gravitational masses revealed by their luminosities.
Generally, such calculations are very accurate, but not with respect to this cluster. Something else was (and still is) present.
Dark matter, which is that obscure something, has been known about and theorized about for almost one hundred years now.
So what is it?
Well, that’s just it. Nobody knows. Sure, there are plenty of cool explanations about it, one of the leading being that it is some type of Weakly Interacting Massive Particle or WIMP for short.
In my opinion, the WIMP theory is the closest to reality, but still falls horribly short when it comes to what dark matter actually is.