Space is mostly empty. We know that. But there is a specific spot in the cosmos that takes "empty" to a level that honestly feels like a glitch in the universe's code. It’s called the Boötes Void.
Imagine you’re driving across a dark, deserted highway at night. You expect to see the occasional flickering gas station sign or the headlights of a passing car. Now, imagine driving for three weeks and seeing absolutely nothing. No lights. No road signs. No other cars. Just a terrifying, oppressive blackness. That is basically what it’s like inside the Boötes Void.
While the rest of the universe is organized into a web of galaxies and gas, this "Great Nothing" is a spherical graveyard of sorts, stretching roughly 330 million light-years across. To put that into perspective, if the Milky Way were sitting in the middle of this void, we wouldn't have even known other galaxies existed until the 1960s. We would have thought we were the only thing in existence.
It’s isolated. It's weird. And it challenges almost everything we think we know about how the universe grew up.
What is the Boötes Void, really?
Back in 1981, an astronomer named Robert Kirshner and his team were doing a survey of galactic redshifts. They weren't looking for a hole. They were just mapping where stuff was. But as they looked toward the constellation Boötes, they realized there was a massive gap where thousands of galaxies should have been.
It’s a "supervoid."
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Most voids in space are small. They are like the little bubbles in a loaf of bread. The Boötes Void is like someone took a giant scoop out of the dough before it finished baking. It represents about 0.27% of the diameter of the observable universe. That sounds small, but in terms of volume, it’s a mind-boggling amount of "nothing."
Standard cosmological models—specifically the $\Lambda$CDM model (Lambda Cold Dark Matter)—suggest that gravity should pull matter together into filaments, leaving empty spaces behind. This is the "Cosmic Web." But the Boötes Void is so big that it shouldn't really exist yet. The universe isn't old enough for gravity to have cleared out a hole that large.
The Weirdness of the Few Galaxies Inside
It’s not technically 100% empty. That’s a common misconception.
Over the years, astronomers have spotted a few lonely residents. We’ve found about 60 galaxies in there. Now, 60 might sound like a lot until you realize that a patch of space that size should normally hold about 10,000 Milky Way-sized galaxies.
It’s a desert.
The galaxies that do live there are fascinating because they are basically "pristine." They haven't bumped into other galaxies. They haven't had their gas stripped away by the gravitational bullying of a cluster. Greg Aldering, an astronomer who has spent a lot of time looking at these loners, once noted that these galaxies are often significantly brighter than the ones we see in our neck of the woods.
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Why? Because they haven't had to share their lunch. They’ve had all the local intergalactic gas to themselves, allowing them to form stars in a way that’s different from the crowded "cities" of the Coma Cluster or the Virgo Supercluster.
One of the most famous theories about why this void is so huge is that it isn't actually one void. It’s probably a bunch of smaller voids that crashed into each other. Think of it like soap bubbles in a sink merging to form one giant bubble. This "merger theory" helps explain the scale, but it still leaves a lot of questions about the dark matter distribution that would allow such a merger to happen so quickly in cosmic time.
Why This Matters for Our Understanding of Physics
If the Boötes Void is "too big," then our math might be wrong.
Cosmologists use the distribution of voids and filaments to measure the "clumpiness" of the universe. If we find too many supervoids, it might mean that dark energy—the mysterious force pushing the universe apart—is stronger than we thought. Or, it could mean that our understanding of gravity on a massive scale needs a total overhaul.
There’s also the "KBC Void" theory to consider. Some scientists, like Ryan Keenan, Amy Barger, and Lennox Cowie, have proposed that our own Milky Way is actually sitting inside a massive, albeit less empty, void. If we are in a void looking at another void, our measurements of the Hubble Constant (how fast the universe is expanding) could be totally skewed.
It’s a bit like trying to measure the speed of a train while you’re walking toward the back of the carriage.
The "Alien Superstructure" Myth
You can't talk about the Boötes Void on the internet without someone bringing up the Kardashev Scale. There’s a popular sci-fi theory that the void isn't natural—that it’s actually a region of space where a Type III civilization has expanded and encased every star in a Dyson Sphere, blocking out the light.
It’s a cool story. It’s also almost certainly wrong.
If there were thousands of Dyson Spheres in the void, they would still give off heat. We would see them in the infrared spectrum. When we look at the Boötes Void with infrared telescopes like WISE or Spitzer, we don't see the heat signature of a trillion alien air conditioners. We see... nothing. It’s just empty.
How to "See" It Yourself
You can’t actually see the void with the naked eye. Even with a backyard telescope, you’re mostly looking at the stars within our own galaxy that are "in the way."
But you can find its location. Look for the constellation Boötes (the Herdsman). Find the bright star Arcturus. The void is located roughly between Boötes, Virgo, and Serpens Caput. When you look at that patch of sky, you are looking at the most desolate neighborhood in the known universe.
Actionable Insights for the Curious
If you want to dive deeper into the mystery of cosmic voids, you don't need a PhD, but you do need to know where to look.
- Track the Cosmic Web: Use tools like the Sloan Digital Sky Survey (SDSS) interactive maps. You can actually see the 3D structure of the universe and spot where the "holes" are compared to the filaments.
- Follow the "Hubble Tension" News: The existence of supervoids is a key part of why scientists can't agree on how fast the universe is expanding. Any news regarding the "Hubble Constant" or "H0" will likely touch on the role of voids.
- Explore the Voids Beyond Boötes: Look up the Eridanus Supervoid. It’s linked to the "CMB Cold Spot," a weirdly cool patch in the radiation left over from the Big Bang. It makes Boötes look like a small pothole.
- Use Amateur Software: Download Stellarium (it’s free). Search for the galaxies NGC 5406 or NGC 5403. These are right on the edge of the void. Seeing how lonely they look on a star map gives you a real sense of the scale.
The Boötes Void reminds us that as much as we’ve mapped, we are still very much in the dark about the "bulk" of our universe. We focus on the stars because they shine, but it’s the empty spaces—the vast, silent stretches of nothing—that might actually hold the secrets to how the whole thing works.
Science isn't just about finding things. Sometimes, it’s about finding nothing and asking why.