No one knows everything for sure about the dark spaces. We have theories. But you're right, Supervoids exist. They are the cold, empty deserts of the universe.
To understand them, you must look back to the beginning. When the universe was young, matter was never perfectly smooth. Tiny ripples. Existed. Gravity began its long, slow work. It pulled things - the heavy parts close together.
These parts became bright stars, they became galaxies. They formed thick webs. Gravity built these structures, but it stole from the other places. The blank parts grew even emptier - A thing called dark energy joined the long fight.
We don't know exactly when. But it pushes everything apart - stretches the void out wide.
Gargantuan. Quiet. Deep spaces.
Gravity won the battle in the bright clusters. The dark voids were left behind.