system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How did we figure out how cold absolute zero is?

From what I understand, it can’t be reached - so how do we know exactly how cold it is?

EDIT: people seem to be misunderstanding, I am NOT asking how cold it is. A quick google search reveals it is about -273.15 C, what I am asking is how we know this for a fact, given it is not reachable.
gpt
Plot the volume of a gas (constant mass) against it's temperature.  As you decrease the temperature the volume will start approaching zero.  The temperature where you would expect an ideal gas to have zero volume is very close to -273C.

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And of course, the more complicated the gas, the more it will deviate from this trend, hence why helium, the most ideal of all real gasses, is the gas we can get closest to absolute zero.