Physicists have created a fluid with "negative mass", which accelerates towards you when pushed.
Usually, when you push matter, it accelerates away from you, thanks to Newton's Second Law of Motion. However, in theory, similar to how an electric current can have a positive or negative charge, there can also be negative mass.
Professor Peter Engels and his team created a Bose-Einstein condensate by cooling rubidium atoms just slightly above absolute zero.
They then trapped the atoms (now acting like waves, as predicted by quantum mechanics) with lasers, and used them to push them back and forth, changing the way they spin. When they were released, they expanded, showing signs of negative mass.
In practise, this would help us explore possible relationships between negative mass and phenomena such as neutron stars, black holes or dark matter.