Many people believe that an object can keep getting colder if you continue cooling it, but that couldn’t be further from the truth. It’s hard to grasp because, in our everyday lives, we don’t encounter extremely low temperatures, nor do we think much about them. However, there is a specific temperature that no matter can go below. To understand why, we need to know what temperature and heat really are.
Temperature is a complex thing to explain. Even though it seems simple, it’s surprisingly difficult to define precisely, so I’ll try to explain it as clearly as I can. When you heat matter, you add energy to the atoms, which increases their velocity. The atoms smash into each other, and this motion generates heat. The hotter something gets, the faster its particles move. Now, conversely, as you cool something down, you decrease the particles' speed. The colder the particles get, the slower they move, and eventually, there must come a point where they stop moving altogether.
Theoretically, though, this is not possible. While you might think back to your science teacher and how they taught you Newton's first law—an object at rest will remain at rest, and an object in motion will stay in motion at a constant velocity unless acted upon by a net external force—when considering things as small as a single particle, you have to refer to quantum mechanics.
I know that seems really complicated, but let me break it down. When something as small as a particle ultimately reaches zero velocity, it still “jiggles” a little. This is called zero-point motion, and it requires energy, meaning it’s impossible to ever reach the coldest temperature possible because that requires no energy at all. You can’t work around it, it's baked into physics as a whole.
There is a point called absolute zero, which is 0 Kelvin (K) or -273.15°C / -459.67°F. As explained before, absolute zero is impossible to reach, but you can approach it without ever getting there. The coldest known place in the universe is the Boomerang Nebula, which sits at around 1 K. We don’t know of anywhere else in the cosmos that cold, and space alone is, on average, 2.7 Kelvin.
In comparison, the coldest place on Earth can get as cold as -92°C, or 181.15 K, which is still far from absolute zero. Nothing on Earth will ever get reasonably close, and if anything, I consider that a good thing. Let's try to stay as far away from those temperatures as possible.