Physicists in the U.S. have created a fluid with “negative mass”—meaning that when you push it away, it accelerates towards you. That means researchers can now use this bizarre phenomenon will now be used to study some of the universe’s biggest mysteries, including dark energy and black holes.

Negative mass is a hypothetical concept that says matter can exist with a mass opposite to normal matter. Instead of having a positive weight—1kg, for example—negative mass would weigh minus 1kg. This works in the same way that an electric charge can be either positive or negative.

In 2014, a team of Canadian cosmologists announced negative matter could indeed exist in the universe without violating our laws of physics. They said as long as negative mass was being produced in a certain way, it could exist according to Einstein’s theory of general relativity.

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Now, Michael Forbes and his team from Washington State University have managed to create negative mass in a laboratory. Their results are published in the journal Physical Review Letters.

Newton’s Second Law of Motion says that if you push something with a given force it will move away at a given speed—depending on the object’s mass, the force and friction. “That’s what most things that we’re used to do,” Forbes explains in a statement. “With negative mass, if you push something, it accelerates toward you.”

To create these conditions, the team used rubidium atoms that had been cooled down to just above absolute zero using lasers. In this state of matter—known as a Bose-Einstein condensate—particles move very slowly and start behaving like waves. The particles move in unison and flow without losing energy, becoming what is known as a superfluid. At this point, the cooled rubidium has a regular mass.

To create the negative mass, the team shot the rubidium with another set of lasers that pushed the atoms back and forth and changed the way they spun. When the “bowl” holding the rubidium they had created smashed, it appeared to have a negative mass.  

“Once you push, it accelerates backwards,” Forbes says. “It looks like the rubidium hits an invisible wall. What’s a first here is the exquisite control we have over the nature of this negative mass, without any other complications.”

Because of this control, scientists can now use it to come up with experiments to test theories relating to extreme astrophysics, such as dark energy and black holes. Previously, scientists have suggested dark matter and dark energy be considered a negative mass. “It provides another environment to study a fundamental phenomenon that is very peculiar,” Forbes adds.