The dark matter (tm).
Probably, I've understood its real nature.
When you are stirring the coffee with a spoon, you can see a typical rotating spiral galaxy in the cup.
It consists of milk particles and random bubbles.
But there is also the invisible component which has its mass and takes part in the rotation.
It's sugar!
The dark matter is the sugar, dissolved everywhere in the universe basin of coffee.
And the galaxies are the local areas in the basin, where every drinker pours the sugar in different amounts (that's why the dark matter distribution is not uniform), stirs with spoon, and then drinks with straw (this explains the "supermassive black hole" in the center of the galaxies, it actually sucks the matter.)
The "big bang", the "universe expansion" are just when they filled the basin with coffee.
We live in a Mad Tea-Party...