Something that our ancestors called the 'aether'.
Scientists proclaimed it didn't exist, now, it is known as 'dark matter' and 'dark energy'.
They say the reason is they can't account for all the matter needed for our Universe to 'exist'.
Seems to me the real problem is they can't SEE all the matter. We are just now starting to recognize that it is possible many stars have planets, maybe all of them. And asteroid belts, and rings around planets.
Maybe there's just a lot of matter there, and we just can't see it.
How long did it take us to find out that Saturn wasn't the only planet in our solar system with rings ?