system
Given the following post, respond as if you're commenting on Reddit:
human
Does energy have gravity?

For something to have gravity, must it have mass? Does light, for example, have gravity associated with it? Would greater concentrations of energy have more gravity?
gpt
Forms of energy which are not mass do indeed contribute to gravity. Light is one example, but most of the time the energy density of light is far too small to have a significant effect compared to all the other stuff in the universe. However, in the very early universe, there was a time when the energy density due to radiation dominated the expansion of the universe.