This is a nice example of a fossil sponge preserved in flint. They are often mistaken for fossil eggs as you suggest, and you can see why!
Flintis formed from a silica gel from the ancient sea floor, and often fossils were preserved in flint when the gel formed a nodule around them because of a chemical attraction to the dead organism. The silica gel later hardened into flint.
Sponges are some of the most common fossils in flint. Sometimes the structure of the sponge is preserved inside the flint but usually the sponge skeleton breaks down and we are left with a hollow spherical flint nodule, known as a “rotten” flint. The fluffy texture is a sign of a “rotten” sponge in flint and isthe remains of the sponge that has broken down.
Your specimen has some of this texture on the inside but the sponge has not completely disintegrated and it looks like you can still see some of the original structure of the sponge. Most spherical flint nodules have this shape because they have formed around a fossil sponge, but fossil sponges in flint can be other shapes as well such as elongated or club-shaped.
As these sponges are preserved in flint we know that they are from the Late Cretaceous, 100-70 million years old , towards the end of the time of the dinosaurs.
Sponges (Kingdom, Animalia; Phylum, Porifera) are animals that live attached to the sea floor. They don’t have mouths or any distinct organs. They pump water through their bodies and filter it to get small particles of food. Sponges have a fossil record that dates back about 600 million years, and there are about 10,000 species alive today. Some are very different to the sponges we use in the bathroom. Most live in the sea but some live in freshwater. They can be very bright colours, such as red, orange, yellow or even blue. Some can be giant, over several metres
I hope this helps, please let me know if you have any more questions.