The theory of general relativity tells us that non-massive entities, and their behaviors, are possible sources of gravity. Mass isn't needed, the theory says.
What's the real-world evidence that non-massive entities, and their behaviors, are sources of gravity? I'm guessing that even if something responds to curved spacetime, that does not necessarily mean it's a source of curvature, itself--?
Also, I do not see how a correspondence between shapes of present-day gravitating structures and CMB radiation inhomogeneities would be an answer to my question. Even during the universe's radiation-dominated era, there was still some matter, which I imagine wasn't perfectly homogeneous (and weren't production/annihilation events happening anyway, generating mass in random spots even as transparency started to take effect?)? So I wonder if mass inhomogeneities were really the only early gravity wells. Not radiation inhomogeneities, per se.
Likewise, what's the real-world evidence that even massive entities, and their behaviors, are sources of gravity when those entities are antiparticles...forgive me for not yet making this an entirely separate Physics StackExchange question...
Thanks very much for your time.