Discovered this large, papery ball under the trees in my yard. It’s light as a feather and completely hollow inside. Is it an egg?

your yard, these structures are a natural part of the lifecycle of certain oak trees and their insect inhabitants.
2. How Oak Apple Galls Form on Your Trees
Oak apple galls begin to form when a female gall wasp lays her eggs on the leaf buds or twigs of an oak tree. The presence of the eggs triggers a reaction in the tree that causes it to produce a growth around the eggs. This growth, or gall, forms over several weeks and serves as both a protective enclosure and a