Abstract entities like Eternity reside on other planes of existence such as Overspace. The exact polar opposite of that would be Underspace – a plane of reality that lies below the dimension called the Microverse. Underspace is the realm Henry Pym placed his headquarters, the Infinite Avengers Mansion, for his team of Avengers because he discovered his former teammate Thor sent Janet Van Dyne, the original Wasp, there.
Pym only revealed the location, and the secret, of Underspace to Jocasta, One-Eyed Jacquie and Ace, of G.R.A.M.P.A., when the quartet of heroes were running for their lives from the murderous entity known as Ultron. After the Skrull invasion, Janet's body was transformed into a living Pym Particle explosion. In the infinite void of Underspace, her new form can continue to expand the same way the universe expands, while the Infinite Avengers Mansion acted in a way to keep her form stable without dissipating. The ten billion Jocastas Pym created performed similarly to the ten billion neurons in the human body - reinforcing Janet's brain wave pattern.
Once Ultron infiltrated Underspace, he temporarily took control of everything, including Janet's mind. Using himself as a distraction, Pym gave Jocasta enough time to override Ultron's programming and regain control of Underspace. With Ultron's defeat, Jocasta confessed she may not have enough power to jump start Jan's mind again, but what ramifications this ultimately has on Underspace remains to be seen.