My cactus, which has a ton of babies on it, has been getting hollow parts on it. Does anyone know why that is? It's healthy otherwise.

Looks like something is eating it, from the growth centers on down.

There are no bugs on it. It's been in the house. I just moved it outside this morning. Several of the babies on the main plant have holes and I repot it today and the main plant also has a hole. It's not rotten though. The roots are fine.