Observable Universe: What It Is, How Big It Is, and What We Know

When we talk about the observable universe, the portion of the entire universe we can observe from Earth, limited by the speed of light and the age of the universe. Also known as the Hubble volume, it’s not the full universe—it’s just the part whose light has had time to reach us since the Big Bang, about 13.8 billion years ago. That means even though the universe might be infinite, we can only see a sphere around us with a radius of roughly 46.5 billion light-years. Why 46.5 billion if the universe is only 13.8 billion years old? Because space itself has been stretching, pulling distant galaxies farther away even as their light traveled toward us.

The cosmic horizon, the boundary beyond which light hasn’t reached us yet isn’t a physical wall—it’s a limit of time and distance. No telescope, no matter how powerful, can see past it. That’s not a technology problem. It’s physics. The farther out you look, the farther back in time you see. The oldest light we detect is the cosmic microwave background, a faint glow from when the universe was just 380,000 years old. That’s the edge of our visual reach.

What’s inside the observable universe? Over two trillion galaxies, each with billions of stars. We’ve mapped galaxies, clusters, and giant filaments of dark matter stretching across billions of light-years. But here’s the twist: most of what’s out there isn’t even made of stuff we understand. About 95% of the universe is dark matter and dark energy—invisible forces that shape how galaxies move and how space expands. We don’t know what they are, but we see their effects. The observable universe is full of mysteries we’re just starting to name.

And here’s the thing—what we see is just a tiny slice. The full universe could be vastly larger, maybe infinite. Some theories suggest other regions of space, beyond our horizon, might have different laws of physics. We can’t prove it. But we can measure how fast space is stretching, how galaxies are moving apart, and how the early universe was structured. That’s how we know the observable universe isn’t the whole story.

What you’ll find in the posts below are real stories from Indian scientists and global researchers who are pushing the limits of what we can observe. From telescope designs that capture ancient light to simulations that model how galaxies formed, these aren’t just theories—they’re active work being done right now. You’ll read about how we measure cosmic distances, why some scientists think the universe might be shaped like a doughnut, and how new instruments might one day peer just a little farther. This isn’t science fiction. It’s science in progress—and you’re seeing it as it happens.