This would be a novel from the 1960s or 70s. Author unknown, title unknown. It might have been designated for young adults, or it might've just been one of those adventure stories that kind of reads like YA.
- The protagonists travelled to an alternate universe/other dimension/etc.
- The people there were having a serious problem with some huge opaque structures of unknown origin, like big domes, that moved slowly across the landscape destroying everything.
- Our heroes managed to get through the surface of one of these, and found that inside it was mostly empty space, with semi-living things scattered here and there, and pools of acid. The big revelation was that this was really a giant single cell containing lysosomes and other organelles.
I don't know what happened after that.