It’s the background quantum field.
It was never compressed by anything else, it’s a phenomena due to the emergence of the particles.
Just like popcorn becomes “compressed” in the bag if it’s in the microwave. As the bag fills, it becomes more and more “compressed”.
The same way the popcorn expands the bag, the quantum field “compresses” the universe and “poofs it up”.
It’s just a continuation of the big bang, which is still going on.
If you use the Planck units to compute the density of the universe it works out to like ten with 109 zeros after it, grams per cubic centimeter.