Anything that has energy (and so far, this seems to be everything that exists), creates a gravitational field. See http://en.wikipedia.org/wiki/Stress%E2%80%93

Anything that has energy (and so far, this seems to be everything that exists), creates a gravitational field. See http://en.wikipedia.org/wiki/Stress%E2%80%93energy_tensor http://en.wikipedia.org/wiki/Stress%E2%80%93energy_tensor