It is known that there are no diproton or dineutron nuclei.
Does this mean that two protons or neutrons are not actually attracted to each other? Even if the attraction was weak, wouldn't it cause bound states anyway?
Related: What do we know about the interactions between the protons and neutrons in a nucleus?
Answer
The nucleon-nucleon interaction has a short range, roughly 1 fm. Therefore if there were to be a bound dineutron, the neutrons would have to be confined within a space roughly this big. The Heisenberg uncertainty principle then dictates a minimum uncertainty in their momentum. This amount of momentum is at the edge of what theoretical calculations suggest the strong nuclear force could successfully fight against. Experiments in 2012 give evidence that the dineutron may be weakly bound, or that it may be a resonance state that is close enough to bound to create the same kind of strong correlations in a detector that you would get from a dineutron. So it appears that the strong nuclear force is not quite strong enough, but this is not even clear experimentally.
If the dineutron isn't bound, the diproton is guaranteed not to be bound. The nuclear interaction is the same as in the dineutron, by isospin symmetry, but in addition there is an electrical repulsion.
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