Physicists at the Ludwig Maximilian University in Germany have hit a temperature lower than absolute zero, rewriting the laws of physics in the process. 

This was achieved through the construction of a quantum gas using potassium atoms, attached to a standard lattice group by magnetic fields and lasers.  The magnetic fields were rapidly adjusted, making the atoms transition quickly from a low energy state to the highest possible state, all while the lasers kept said atoms in place.  With this negative temperature state, the gas dipped to "a few billionths of a Kelvin below absolute zero."

With this breakthrough has come some rather interesting side effects, leading to potential "new forms of matter."  For example, while atoms are usually pulled downward by gravity, those below absolute zero float upwards instead.  They also show characteristics much alike dark energy, which is what is (hypothetically) responsible for the expansion of the universe.  It's an exciting discovery, that pushes us beyond the theoretical 'walls' of physics.

Source: Negative Absolute Temperature for Motional Degrees of Freedom

Jason England