Complicating this already-mind-boggling question is the fact that our best theories conflict with our observations of the universe. Albert Einstein, according to scientific folklore , felt a unique responsibility for introducing this entire problem, reportedly referring to it as his "biggest blunder."

A persistent cosmological puzzle has been troubling physicists since 1917: what is the universe made of?

What was previously assumed to be empty space in the universe now had to be filled with huge amounts of mysterious anti-energy in order to explain observations of the universe’s ever-quickening expansion. Even so, observations of the universe’s expansion suggest that the energy is 60 to 120 orders of magnitude lower than what recent quantum field theory predicts.

This proposal worked when considering small regions of spacetime. But when Einstein applied it to the entire universe, its predictions didn't fit. So, Einstein introduced the "cosmological constant," a fixed value that represents a kind of anti-gravity, anti-mass, and anti-energy, counteracting gravity’s effects. But when scientists discovered that the universe was expanding rather than static, as Einstein had believed, the cosmological constant was set to zero and more or less ignored. After we learned that the universe’s expansion is accelerating, however, scientists could no longer conveniently cancel out Einstein’s anti-gravity suggestion.

Essentially, Einstein's novel theory of general relativity didn’t hold up when used to describe the universe as a whole. General relativity described the "geometry" of spacetime as being a trampoline-like surface; planets are heavy bowling balls that distort the surface, creating curves. If a less heavy ball (like a marble) was placed near the bowling ball, it would roll along the surface just like the motion of planets in orbit. Thus, orbits are explained not by a gravitational “force” but by curvature in spacetime.

What this means is that all of this extra energy is somehow missing when we look at the universe as a whole; either it’s effectively hidden or very different in nature to the energy we do know about.

Today, theoretical physicists are trying to reconcile these mysteries by examining the structure of so-called “spacetime” in the universe at the smallest possible scale, with surprising findings: spacetime might not be the trampoline-like plane scientists once envisioned—it might be a foamy mess of bubbles all containing mini-universes living and dying inside our own.

What is spacetime foam?

To try and solve the mystery of what fills the universe, scientists have been exploring the possibility that it's actually full of bubbles.

In 1955, influential physicist John Wheeler proposed that, at the quantum level, spacetime is not constant but "foamy," made up of ever-changing tiny bubbles. As for what these bubbles are "made" of, recent work suggests that spacetime bubbles are essentially mini-universes briefly forming inside our own.

The spacetime foam proposal fits nicely with the intrinsic uncertainty and indeterminism of the quantum world. Spacetime foam extends quantum uncertainty in particle position and momentum to the very fabric of the universe, so that its geometry is not stable, consistent, or fixed at a tiny scale.

Wheeler illustrated the idea of spacetime foam using an analogy with the surface of the ocean, as retold by theoretical physicist Y. Jack Ng at the University of North Carolina, Chapel Hill, in an email:

Imagine yourself flying a plane over an ocean. At high altitudes the ocean appears smooth. But as you descend, it begins to show roughness. Close enough to the ocean surface, you see bubbles and foam. Analogously, spacetime appears smooth on large scales; but on sufficiently small scales, it will appear rough and foamy.

Professor Steven Carlip at University of California, Davis, published new research in September that builds on Wheeler's quantum foam theory to show that spacetime bubbles could “hide” the cosmological constant at a large scale.