One thing we know for sure is that light is a kind of wave, like the waves you see when you throw a stone into a pond. For a long time people thought that this meant that space was filled with an invisible fluid and that light was just ripples in that fluid. They even gave the fluid a name, they called it 'aether' (pronounced EE-ther). But no matter how hard scientist tried they couldn't find any evidence that aether was real. Later, Albert Einstein and other scientists decided that there's no such thing as aether after all. Light moves like a wave, but through empty space! So remember, just because something has a name doesn't mean it's real.