Well, yes, as far as our theories go. But we also “knew” that light was a wave that traveled through the luminiferous aether, which permiated all of space… Until we tested that theory with the Michelson-Morely experiment, and it turned out our theories were completely wrong and physics as we knew it was completely upended.

Point being, it’s important to actually test our theories instead of assuming they’re completely correct just because most of their predictions are accurate.