If negative matter naturally exists where would it go?

In our universe matter attracts itself via gravity and lumps into stars, planets and toaster ovens. As I understand it negative matter would be anti gravity and would be repelled, thus prevented from doing this?

Would negative matter attract itself via gravity caused by other negative matter? And where would it end up in the universe? Just mostly floating around in the intergalactic void?

I’ve read the hypothesis that, upon the birth of the universe, there was almost as much anti-matter (negative protons & positive electrons) as there was matter, and they annihilated. The survivor was matter. It also explains why the vast majority of known universe is virtually devoid of matter.

Thank, yes I have heard that, but I am asking about negative matter, not antimatter. They are not the same thing.

You surely know the image where space is depicted as a two dimensional sheet that gets deformed by tiny balls representing matter. If negative matter existed (which I doubt), in this image it would act like helium balloons on this sheet, but stiking to it, as they could not leave space. So they would naturally accumulate there where the sheet is highest and would even pull it up, forming a dome where normal matter would producce an indentation. The more balloons there were, the more they would attract additional balloons, if there were any close.
But how would this negative matter work? If you know the principle of equivalence, you cannot distinguish the action on gravity from the uniform acceleration produced by another force. Would that mean that your postulated negative matter would move in the direction opposed to the force exerted on it? The more I push it, the more it pushes back? Or would this matter allow us to finally distinguish gravity from other forces? It looks like this negative matter leads to contradictions, I doubt it exists.
OTOH, seeing a contradiction in action would be fun.

Thanks, I love that analogy. The take away from that however is that negative matter would likewise clump up. I’ve always assume it would repel even other negative matter as thus disperse itself as if those balloons statically repelled each other, so they would group in the voids however with vast spacing still between itself. Though I guess it could lump together and maybe form negative stars etc.

Perhaps the better image would be that the balloons are on the underside of the blanket. Make the blanket, the two dimensional visualization of the three dimensional space, transparent to see both, or just slightly tinted.
The analogy is flawed anyway because the image is used to explain gravity on the blanket but relies on gravity in the room to make those indentations.
Still doubt they exist.

PBS Spacetime episode on negative mass (which I guess is the same thing as negative matter).

As they explain, it leads to all kinds of weird effects; like that a negative and positive mass apple coming close would accelerate each other forever; the positive apple heading towards infinite kinetic energy and the negative apple towards negative infinity kinetic energy.

You could also trivially make a perpetual motion machine, assuming you had a methodology to put the negative mass where you need it in the first place.

Probably anyway.
The end of the video concedes that there is still debate on this, as some theorists think that there may be flavors of negative mass that could hypothetically exist without breaking known physics.

The rubber sheet analogy is severely limited, and this is one of the limitations. You can’t derive the properties of negative matter from the rubber-sheet analogy. Of note, if I had a negative-mass “apple”, and released it near the surface of the Earth, it would fall downwards, just like an ordinary positive apple. The difference between the two would be what the Earth does in reply: Instead of falling upwards (ever so slightly) to meet the apple, it would fall downwards away from it. And while this downward motion would be very small on the scale of the Earth, for two similarly-sized objects, you’d get the infinite acceleration @Mijin mentioned.

I think I don’t really understand why negative mass is so troublesome, when we have positive and negative charges and magnetic poles that repel or attract with no issue. Is this a general relativity thing having to do with the warping of space time, which (I guess) E&M fields don’t do?

The trouble is that mass has two “purposes”. It is a gravity charge (but so far only of the positive kind) just like electrical charges and magnetic poles are. But it is also in the F = ma equation. In a pure Newtonian world, you could have, I’m pretty sure, negative mass and then write F = |m|a and things would be fine. Some masses would pull others and some would repel.

The problem with negative mass comes in the F = ma equation. If you push on a mass it doesn’t accelerate in the direction you’re pushing but in the opposite direction as the negative sign flips the direction of the vector. One premise of General Relativity is that you cannot distinguish acceleration from a gravitational force, so you can’t have F = |m|a. That’s when you get the negative and positive masses “chasing each other” and increasing their mutual speeds.

OK, mind blown. I had forgotten about GR equivalence thing. Thanks for the explanation, which I sort of understand.

The biggest problem is that the gravitational “charge” also functions as inertial mass, the m in F = ma . That means that if you put a leftward force on a negative-mass object, the object responds by accelerating to the right. Even without gravity, that sort of behavior would lead to a lot of problems.

Antimatter is matter with charge opposite to that of normal matter. Its mass is not negative.