I understand that with a hollow cube with the inner walls covered in mirrors given a light source briefly, the light would eventually be absorbed. This is due to electron excitation I believe. So suppose the cube had walls of positrons instead of mirrors, since there are no electrons what would happen?
I'm imagining a box made completely out of anti-matter so that your situation is realistic.
Positrons are the antiparticle of the electron (i.e., the anti-matter equivalent). Meaning, in this case, they're identical to electrons except for charge.
Photons, though, have no charge. So don't give a hoot whether a charged particle it's interacting with is positive or negative; the resulting interaction will be the same regardless.