Hello everyone! Just something I was thinking about today that I haven't been able to shake. I recently read that space is not actually a perfect vacuum, but that it contains low density particles, plasma, electromagnetic fields and so on. Is 'hard vacuum' the correct definition, or is it okay just to use vacuum? I'd like to know why these low density particles, fields, and plasma don't interact with the light or slow it down in any way (or do they?) While I'm on the subject of particles, do virtual particles exist in space, and do they interact with light? Thanks!