A Glassverse is a type of verse composed entirely of glass (except on the areas that there are Xenoverses), hence its name. It is extremely fragile by cosmic standards (comparable to level 8 glass or White House window glass), so it contains Xenoverses to help the glass not break.
Every times a crack occurs, the Xenoverses rush over to bind the fallen glass particles together. If the glass does break, and no Xenoverses come to fix it a, gigantic hole has a somewhat high chance to form and suck everything inside the affected Glassverse in a 1 trillion light year radius and forcing it to be thrown out into god knows what, but chances are that a Glassverse reaches this point won't happen ever naturally.
Clusters of Xenoverses usually crowd where cracks once formed, which leads to thousands of clusters sitting there waiting to fix the next crack that may form there. Xenoverses are so used to covering up cracks, that there are thousands of voids but little amounts of Xenoverses chill out in the middle of no where.