Physics Discussion

This question has been bugging me:

Does the light add to the gravity of a given volume of space?

Transmission of light appears to be affected by gravity so it seems logical that should exert a foce in the other direction but can it be proven (i.e. measured)?

I know most people regard the "rest" mass of a photon as 0 but what about a photon in motion?

This thought came up while thinking about how the mass of the universe is calculated and wether the mass/energy of light is taken into account.

Light has energy, which is equivalent to mass, and so it has a gravity. A volume of space which has energy, light or otherwise, will have a gravitational effect on other objects. The rest mass of a photon is zero, but a photon of energy E has a mass of E/c^2.

The contribution of light in calculating the mass density of the Universe, as far as I know, is too low compared to the visible baryonic matter, and the invisible dark matter. And now, it seems that dark energy contributes hugely to the mass density of the Universe.