Imagine removing everything from the deepest regions of cosmic voids. Take away ordinary matter, neutrinos, dark matter, cosmic rays, and radiation. What remains appears to be nothing but empty space. It may sound contradictory, but these enormous voids are filled with the vacuum of spacetime. And importantly, that vacuum is not truly nothing.
The vacuum of spacetime contains something fundamental. It is difficult to describe precisely with everyday language, but physicists refer to these underlying ingredients as quantum fields. In quantum field theory, the particles that make up our world such as electrons, top quarks, neutrinos, and even dark matter are not independent objects in the usual sense. What we call a particle is actually a visible expression of something deeper.
These deeper structures are the fields themselves. Every type of particle has a corresponding field. These fields permeate every cubic centimeter of space and time. They have existed since the big bang and extend throughout the entire universe.
When we observe a particle, such as an electron moving through space, we are really detecting a ripple or vibration in its underlying field. The particle is a traveling excitation of that field. Even if all particles were removed, the fields would still remain.
Vacuum Energy and the Origin of Dark Energy
These fields also contain energy. Because of the Heisenberg uncertainty principal, the vacuum cannot be completely devoid of energy. When physicists attempt to calculate how much energy exists in empty space, the results can range from extremely large values to theoretically infinite ones…which is also another episode.
What matters is that this vacuum energy produces a measurable effect. That effect is known as “dark energy,” the name scientists use to describe the accelerated expansion of the universe.
Observations show that the actual amount of vacuum energy is relatively small, though it is not zero. In most environments across the universe, its influence is negligible. Regions filled with matter completely dominate the local behavior of space.
Here on Earth, for example, matter is so dense that dark energy has no noticeable impact. If dark energy suddenly vanished, everyday physics would remain unchanged. The path of a thrown baseball would be identical. Your burrito would still cook in the microwave at exactly the same rate. Nothing about daily life would be different.
Where Dark Energy Dominates the Universe
The same situation applies across much of the cosmos. Galaxies, galaxy clusters, filaments, and walls of the cosmic web are all regions packed with matter. In these environments, dark energy plays almost no role.
Cosmic voids are different.
Voids are enormous regions where matter is largely absent. In these areas, the vacuum of space-time itself becomes the dominant influence. If you could place yourself in the middle of a cosmic void, you would effectively be surrounded by dark energy.
In fact, voids are where dark energy carries out its most important work. The accelerated expansion of the universe does not occur inside dense regions such as galaxies or clusters. Instead, it takes place within the vast empty voids.
Cosmic Voids Are Expanding
Cosmic voids are not just empty gaps between structures in the universe. They are actively growing. As dark energy pushes space outward, the voids expand and press against the surrounding cosmic web.
Over immense spans of time, this process gradually pulls the universe’s large-scale structure apart. The intricate network of galaxies, clusters, and filaments that astronomers see today will not last forever. Over the next 5-10-20 billion years the exact number doesn’t matter the cosmic web will slowly fade as expanding voids stretch everything farther apart.
Why Empty Space Is Never Truly Empty
In that sense, cosmic voids are far from empty. They are filled with the subtle energy of quantum fields. That energy influences the entire universe by driving its accelerating expansion.
Voids are the only regions where this effect becomes dominant, precisely because they contain almost nothing else.
So yes, cosmic voids are empty of matter. That is how astronomers identify and measure them. But their lack of matter means they are filled with dark energy.
Wherever you travel in the universe, whether to a nearby galaxy or to the deepest interior of the emptiest void, you will never truly be alone.