A cosmic void is a huge region of the Universe where galaxies and matter are much less concentrated than average. They are not completely empty. Extremely thin gas, dark matter and occasional galaxies can still exist inside them. Voids are surrounded by denser structures such as galaxy filaments, sheets and clusters.
On the largest scales, matter in the Universe forms a vast network known as the cosmic web. Galaxies gather along enormous filaments and walls, while huge empty regions form between them. Gravity gradually pulled matter toward denser regions, leaving some areas with far fewer galaxies.
No. A cosmic void is not a perfect vacuum. It can contain extremely diffuse gas, dark matter and isolated galaxies. The important difference is density: there is dramatically less matter than in the surrounding cosmic structures.
Tiny variations in the density of the early Universe grew under gravity. Matter flowed away from underdense regions and toward denser ones. Over billions of years, these differences became the enormous network of clusters, filaments and voids that we observe today.