How can photons be caused to be excited in the space of the Universe?

Answer 1

Atoms in the stars surfaces are primarily excited by the huge energy of nuclear reactions inside of stars. Excited atoms emits photons. Then those photons interacts with the hot matter of the surface of the stars, producing thermal radiation.

Atoms in dust clouds of interestellar space are excited by photons or particles coming from the stars, absorbing the original photons and producing new ones. Sometime they simply absorb those photons and then we observe what is called the absortion spectra.

Answer 2

English is so imprecise.
It is not clear what you mean by excited.
There are several problems with which you could be concerned.
Are you concerned about the apparent creation of photons from nothing in otherwise empty space?
Are you concerned about the ability of electromagnetic waves to propagate across otherwise empty space?
Are you concerned about the particular energy content of photons that are created by chemical processes?
Are you concerned about the particular energy content of photons that are created by nuclear processes?
Each of these questions could be the basis of a very respectable undergraduate senior thesis.

Answer 3

If the vacuum where actually a false vacuum that was (by means not understood by modern physics) decaying into a lower energy false vacuum, then it might spontaneously emit photons.

Answer 4

If it sees the hottest opposite sex walking by. Good enough, yes?