I was reading this wikipedia article on positronium (a type of exotic atom) and noticed it mentioned that positronium has a spectra (of course).
So naturally this makes me wonder, have we looked at stars or other glowing objects to try to detect spectra of exotic atoms? It seems plausible to me that there would be extremely faint but non-0 spectra of these exotic atoms within the stars, or maybe in the center of the galaxy.