I am just a physics layman, but I recently watched a documentary about the universe and it was told that
- the universe is full of dark matter and energy and
- the universe is empty, so that the average temperature measured by the cosmic microwave background is about 2.73 Kelvin.
They also said, that temperature is (also) depending on gravity or mass respectively. That's why the sun is so hot, since there is huge mass concentrated, and the empty space is cold, since there is no mass. Or is it?
So now my question(s): IF there is almost only dark matter and energy in the universe with HUGE mass effects which explains interstellar movement etc., why is the universe only 2.73 K cold? Shouldn't it be warmer? Isn't the 2.73 Kelvin the temperature you get, if you calculate the temperature without dark matter and energy?