That destroys the Universe
Hohmann 16 – Creator and Editor of Moonwards, “Tempertures Below Zero Degrees Kelvin”, World Building, 5-29, https://worldbuilding.stackexchange.com/questions/42510/tempertures-below-zero-degrees-kelvin
Temperature is a measure of how much the atoms vibrate and move (e.g. kinetic energy). 0 Kelvin is defined as the point where the atoms do not vibrate any more at all, so you can not really have sub-kelvin temperatures, as it is not possible to move less than standing totally still. If you somehow discover that atoms at what we now believe is absolute zero actually move a little, your new temperature is just defined as the new 0 Kelvin.
A second consideration is that negative degrees Kelvin allows for heat flow that reduces entropy, a pretty important violation of physics, as that allows for infinite energy, perpetual motion machines, etc.
In short, negative absolute temperatures can not exist in reality, and in a sci-fi setting, it is likely to have a lot of unexpected consequences, having a good chance to somehow destroy the universe.