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).

At the start of the process, mass is converted into energy and emitted as sunlight. At the end of the process, photons lose their momentum by converting it back into mass as they slow down. The universe has been shining for around 14 billion years - that's a lot of photons and a lot of mass forming invisible spheres around the galaxies.

Dark matter = stationary photons.

Obvious really.