That’s such a widely used concept and it’s erroneous. You can’t ALWAYS prove a negative. But if you’re able to prove a mutually exclusive positive to the negative condition, then you’ve proven it. For example, proving it is daytime where I’m standing also proves it is not nighttime where I’m standing.
There are circumstances where a negative cannot be practically proven, or without an absurd amount of work. But all you really need to do is empirically demonstrate the negative is the likeliest reasonable scenario and that’s usually good enough, except to someone obstinately trying to stay with their position and therefore demands absolute unequivocal proof - which is a rarity entirely.