Yes, we know that’s an odd question, and yes, we know not just the universe, it’s impossible to precisely count the atoms in pretty much anything. But that title got you thinking, didn’t it?

We often think we have answers to the mysteries of the world, but that's not true. No matter how many degrees we bag or how much research we do, there will always be certain things that would remain unknown to us.

Speaking of unknowns, do you know how many atoms there are in the universe? Well of course, like we said earlier, it would be impossible to put an accurate number to it, but what’s surprising is that scientists have found a way to “estimate” it at least. Is there anything these guys can’t do?!

## Starting with the basic question – how big is the “observable” universe?

The human body alone has around seven octillion atoms. Now you might be thinking, "if our body has so many of them, wouldn't it be impossible to calculate the number of particles in the universe?" TBH, it would be! That's why we aren't going after the whole universe; we're only sticking to the part we as Earthlings know about, i.e., the observable universe.

The Big Bang created the universe 13.8 billion years ago, and since then, it has been expanding outwards. Earlier, people thought that it only stretched 13.8 billion light-years in every direction. But they’ve been proved wrong. When they understood that by the time they observed a star or galaxy, it had already moved from its place, they went with what the scientists' best guess was - 46 billion light-years in every direction.

But sadly, knowing the size of the observable universe isn’t enough to find the number of particles in it. We also need to find how much matter exists in that part.

## So, how much matter is there in the observable universe?

According to NASA, there's only 5% matter in the universe. If we go by Einstein's famous equation, E=mc^{2}, which states that energy and mass are interchangeable, we can assume that the quantity of matter, created and uncreated, cancel each other. That means mass is finite, and it has been the same since the day the universe was created. In other words, the number of atoms has also been the same.

But at this point, we have to make two assumptions to get down to the calculations. First, we need to restrict ourselves to including only stars into the equation as we don't know about the other objects. And second, we need to assume that all the stars are made up of Hydrogen atoms since that will make our calculations easier.

## And now for the final calculations

According to European Space Agency, there are 10^{11} to 10^{12} galaxies in the observable universe, and each galaxy has 10^{11} and 10^{12} stars. If we multiply the two, we get 10^{22} and 10^{24} stars. Taking 10^{23} as an average and multiplying it with the weight of one star (2.2x10^{32} pounds), we get the universe's mass as 2.2x10^{55} pounds, and that comes down to how many atoms fit into that mass.

As per Fermilab, one gram of matter has 10^{24} protons. Since we’re considering only Hydrogen atoms and each Hydrogen atom has one proton, the number remains unchanged. Now multiplying this with the universe's matter, we get 10^{82} atoms in the observable universe.

If you got all that, you’d agree that that is a figure most of us can't even fathom yet. TBH, most of us won't even understand those hefty calculations. But, it sure makes for an excellent question to throw at people at social gatherings, doesn't it?