Space is big. You know that. It’s the kind of thing we’re told in third grade while staring at a plastic model of the solar system, but the model is a lie. If the Earth were the size of a marble, the Moon would be a pea thirty feet away. But the Sun? That would be a giant yoga ball two miles down the road. And that’s just our backyard. When people ask about the size of space, they usually mean one of two things: the part we can see, or the part that exists beyond the "curtain."
It’s huge. Honestly, it's bigger than that.
The Observable Universe is a sphere centered on us, stretching about 46.5 billion light-years in every direction. That gives us a diameter of roughly 93 billion light-years. But wait. If the universe is only 13.8 billion years old, how can it be 46 billion light-years wide? Light hasn't had time to travel that far, right? This is where the physics gets weird. While light is traveling toward us, the space underneath that light is stretching. It’s like trying to run on a treadmill that is growing longer while you sprint. By the time the light from a distant galaxy reaches your telescope, that galaxy has been pushed much further away by the expansion of the universe.
Most people think of space as an empty room. It isn't. It's more like a fabric that can bend, twist, and grow.
The Edge You Can’t Ever Reach
The "Observable Universe" isn't a physical wall. It’s a limit of information. We are stuck inside a bubble of light. Anything outside that bubble is moving away from us faster than light can travel. This doesn't break Einstein's rules, by the way. Nothing can move through space faster than light, but space itself can expand at whatever speed it wants.
Because of this, there are parts of the universe we will never, ever see. Even if we built a ship that could travel at 99.9% the speed of light, we could never catch up to the galaxies at the "edge." They are effectively gone. For all of human history, they are ghost stories told by math.
Is there an end to it? Cosmologists like Max Tegmark or Alan Guth have spent careers arguing over whether the universe is finite or infinite. If the universe is "flat"—a term scientists use to describe its geometry—it could go on forever. Imagine an infinite sheet of paper. If you keep walking, you never loop back. Recent data from the Planck mission, which mapped the Cosmic Microwave Background (the "afterglow" of the Big Bang), suggests the universe is incredibly flat, with only a 0.4% margin of error.
If it’s truly infinite, that leads to some terrifyingly cool outcomes. In an infinite universe, every possible arrangement of matter must happen. Somewhere, there is another version of you reading this exact sentence, except maybe you’re wearing a green hat. Or you're a dolphin. Infinity is messy like that.
Measuring the Void Without a Ruler
How do we actually know the size of space without going there? We use something called the "Cosmic Distance Ladder."
📖 Related: Satellite North Korea at Night: The Truth Behind That Famous Dark Map
- Parallax: For things that are relatively close, we use basic geometry. As Earth orbits the Sun, we look at a star in January and then again in July. The star appears to shift slightly against the background. It’s the same effect as holding your thumb out and closing one eye, then the other.
- Cepheid Variables: These are stars that pulse like a heartbeat. In 1912, Henrietta Swan Leavitt discovered that the brightness of these pulses is directly related to how much energy the star is putting out. If you know how bright it actually is and compare it to how dim it looks, you can calculate the distance. This is how Edwin Hubble realized the Andromeda "nebula" was actually a whole other galaxy.
- Type Ia Supernovae: These are our "standard candles." These exploding stars always blow up with roughly the same amount of energy. They are so bright we can see them across the universe.
When we look at these markers, we see the "Redshift." Light from distant objects gets stretched out into longer, redder wavelengths. It’s the Doppler effect, like a siren dropping in pitch as an ambulance speeds away. Except here, it’s galaxies speeding away. And the further they are, the faster they go.
The Unseen Bulk: Dark Matter and Energy
We can't talk about size without talking about what's filling the volume. If you took everything we can see—stars, planets, black holes, your cat—it only makes up about 5% of the universe. The rest is Dark Matter (about 27%) and Dark Energy (about 68%).
Dark Matter is the invisible glue holding galaxies together. Without it, galaxies would fly apart like mud spinning off a tire. Dark Energy is the opposite; it’s the foot on the gas pedal. It’s what is causing the expansion of the universe to accelerate.
We are living in a weird era of cosmic history. Billions of years ago, the universe was dense and crowded. In the far future, Dark Energy will have pushed every other galaxy so far away that the night sky will be completely black. Future astronomers on Earth (if it still exists) won't even know other galaxies exist. They’ll think they are alone in a tiny, dark island of stars. We’re lucky. we got here while the lights were still on.
Visualizing the Impossible
To grasp the size of space, you have to stop using miles or kilometers. They’re useless. A light-year is about 6 trillion miles. The Milky Way is 100,000 light-years across.
Think about the Hubble Extreme Deep Field image. Astronomers pointed the telescope at a tiny, dark patch of sky—about the size of a grain of sand held at arm's length. They left the shutter open for weeks. When the image came back, that "empty" patch was filled with 5,500 galaxies. Each galaxy has billions of stars. And that was just one grain of sand. There are millions of those "grains" across the entire sky.
Some theorists, like those working on String Theory, suggest our 3D universe might just be a "brane" floating in a higher-dimensional space called the "Bulk." If that’s true, the size of space we talk about is just the surface of a much deeper, more complex ocean. But that's still mostly math and speculation. For now, we stick to what the photons tell us.
What You Should Actually Do With This Information
Knowing the scale of the cosmos is fun for trivia, but it actually changes how you look at modern technology and exploration.
- Stop waiting for "Warp Drive": Understand that based on our current grasp of physics, interstellar travel is a generational commitment, not a weekend trip. Even at the speed of light, it takes four years to reach the nearest star.
- Follow the JWST: The James Webb Space Telescope is currently looking at the very first galaxies to form after the Big Bang. It is essentially a time machine looking back 13.5 billion years. Watching its data releases is the closest you’ll get to seeing the "edge."
- Check out "The Scale of the Universe 2": There’s a fantastic interactive tool online (often found on sites like HTwins) that lets you scroll from the size of a quantum string all the way to the observable universe. It is the best way to feel the "cosmic vertigo."
- Support Light Pollution Reduction: You can’t appreciate the size of the universe if you can only see three stars. Join the International Dark-Sky Association or just use shielded outdoor lighting to help keep the "window" open.
The universe is expanding, and it’s doing it faster every second. Every day, the total amount of the universe we can actually reach gets a little bit smaller. It’s a big place, but it’s also a disappearing one.