Well I want to have a part that is super buoyant in the water that is capable of holding up a unrealistic amount of weight But the max density is 0.01 witch does not offer much. Pretty much I want to keep a part buoyant whitest having a crazy mass of 100 on top of it.
You want to simulate wormholes and Dark matter? A negative density would mean a negative weight. I think what you mean to ask is to have the lower threshold of 0.01 moved closer to 0 instead.
EDIT: now that I think about it, you could probably do your use case better with body objects?
Why not reduce the density of the heavy part(s)?
Got to admit, that’s a pretty awesome idea.
Not sure how the part would behave though.
Negative weight, so I assume the gravity is also negative for it, meaning it goes up.
Still lower density than 1 (water), so it would also go up in water (pretty fast).
But what if it collides with other parts?
Will the other part accelerate instead of bounce?
Will the part itself have its force *-1 'd?
Ah right… Not dark matter lol
Allowing a negative density makes no logical sense.
It would mean that either the mass or volume of the object is negative, and neither of those variables can be defined as negative.
Well they can put the weight as negative in the physics engine. But it would probably have some very interesting effects. 
(For example, when two bodies collide and one of them has negative mass, then they may go through each other instead of bouncing off of each other. And gravity would be inversed, and the effect of body objects would be inversed too.)
Negative density implies negative mass.
So when anything hits your negative mass object it negates the direction on the force vector according to F = MA.
This will probably not give the result you are looking for.
Buoyancy is completely based on the amount of water displaced vs the amount of weight of the objects displacing it. If you want to carry heavier objects you basically have to make them take up much more space in water. Make a larger 0.01 Density platform. This is how boats work. Eventually if you put too much heavy crap on a boat, it will sink.
You could also use BodyMovers, depending on what you’re trying to do.
Overlap multiple low density parts. Roblox bouancy is per-part (ill just assume this is true…), so even though the occupied volume is the same (looking at the whole thing), technically youre increasing displaced volume = increasing boyancy.