Question: what is a big scientific mystery

Geraint Evans answered on 16 Mar 2021:

Dark Matter and Dark Energy are two very big mysteries about the universe that we can’t answer.

According to what we know, galaxies and solar systems are spinning faster than you would expect for how much stuff is in them. Usually, the more stuff you have the faster you spin, so there must be extra “stuff” in around these stars and in galaxies that we can’t detect. We call it Dark Matter (because we can’t “see”/find it).

The universe is expanding faster than it should. Scientist expect the universe to slow down, but the opposite is happening, it’s speeding up! So, like dark matter, there must be something else we can’t find yet accelerating the expansion of the universe. We call whatever it is that is causing this unexpected behaviour Dark Energy.