Dark matter

Darkness = absence of light.

Light = moving photons.

Photons are very weird things (some might even say strange). Sometimes they act like particles, sometimes they act like waves. They have no mass when they are moving, even though they are deflected by gravity, but do have momentum which can be transferred to physical objects (like solar sails).

Stationary photons lose their momentum by converting it into mass - and after 14 billion years of the light of the universe, that's a lot of photons and a lot of mass.

Dark matter = stationary photons.

Obvious really.