A question about light and the medium it propagates through.

Maybe I’m scientifically illiterate and I really hope to clear this up for myself because I’m sure it’s obvious, but since light is a wave, then what is the medium that’s being oscillated? This kind of suggests that the entire universe is made up of it considering light can travel anywhere. So what exactly is it, and so we understand it well? (I want to apologize if my question is trivial and obvious, to me it’s quite hard to wrap my head around)…