To understand how small the universe really is, you must first understand how large it is — it’s larger than you think, if you haven’t read about the expansion of space. The best current estimate for the age of the universe is just under 14 billion years, so the simple value for the size of the observable universe would be just under 14 billion light years in every direction from Earth. With expansion it’s much larger than that: about 46 or 47 billion light years in any direction, or 93 billion light years across. I can ride a bicycle 100 miles in a day — it would take me 15,000,000,000,000,000,000 years to ride across the universe. I’d better get started.
Translate that into volume and the number gets crazy. If our solar system has a radius of 124 astronomical units (that’s how far out Voyager I is about…now) then the universe is about 13,000,000,000,000,000,000,000,000,000,000,000,000,000 times the volume of our solar system.
Okay, so the universe is really big, right?
In fact, it’s quite the opposite. Allow me to explain. The estimate for the mass of the universe ranges from about 8×10^52 kg to just under 4×10^54 kg. To quote Marty McFly, “Whoa. This is heavy.”
But not heavy enough. The universe is almost unimaginably sparse. There’s a lot of space, but not a lot of stuff. If you gathered all the mass in the universe together, it would be incredibly large in terms of days to bicycle it, but incredibly small compared to the space it normally occupies: the universe.
For comparison, the nearest star to our sun is Proxima Centauri, a little more than four light years away. Round that up to five light years. Fill a sphere with water, centered on our sun and with radius 5 light years, and in addition to having the deepest swimming pool in history, the resulting mass would be about 4.3×10^53 kg, or about the same as the mass of the observable universe. So:
Everything we know in the universe — every planet, every star, every galaxy, the dark matter, the dark energy, all of it — weighs about the same as a sphere of water that barely reaches our nearest neighbor star.
That sphere is large compared to us — it would take about 1.6 billion years to bicycle across it — but it’s really small, even compared just to our galaxy. If you were looking at an image of the Milky Way on your monitor, you wouldn’t be able to see that ball of water. Yes, even with a retina display. And compared to the universe, the sphere ridiculously small. The universe could contain about 1,000,000,000,000,000,000,000,000,000,000 of those spheres. Not so big, eh? Another way of looking at it is to say that on average the universe is 1/1,000,000,000,000,000,000,000,000,000,000 as dense as water.
Note that if this hypothetical sphere of water suddenly popped into existence, it would immediately start collapsing in on itself at near the speed of light. At the surface — 5 light years away from the center of mass — the water would experience a gravitational force roughly a billion times that at the Earth’s surface. Did I say immediately? Ignoring relativistic effects, it would take the water at the surface about a thirtieth of a second to accelerate to near light speed.
I’m not sure what would happen to the sphere of water as it crushed itself, but I’d be happy to observe from a distance — let’s say a few billion light years away, just to be safe.
You might be thinking about calling shenanigans, based on those really big stars like NML_Cygni. It’s the largest known star: if it were put in place of our sun, it would engulf Jupiter, but not reach Saturn. But for all its size — about 5 billion times the volume of the sun — it has only about 40 times the sun’s mass. At its surface, it’s more vacuum than matter. It might be hard for NML Cygni to burn you using thermal heat alone, because it’s so thin. Don’t try it though, the radiant energy would easily cook you.
Water is great for comparison, but at a molecular level it’s still mostly empty space. To pack a little tighter, consider the density of a neutron star, generally over 1 billion times the density of water. So if you pack it into one big nuclear mass, the entire universe would fit in a sphere only about 300 astronomical units in radius — give Voyager I a couple hundred years and it will have traveled that far. The gravity at the surface of that sphere would be almost unimaginable.
If we traded the current universal setup for either of these spheres, the entire rest of the universe would be empty — completely empty. Of course, it’s very nearly empty now.