Even if antineutrons were stable, mixed with normal matter they will colide with protons and release a poiton and a lot of energy. (And the positron annihilate

Even if antineutrons were stable, mixed with normal matter they will colide with protons and release a poiton and a lot of energy. (And the positron annihilate with an electon.) Anyway, I think there is still a chance that dark matter is antineutrinos. I'm not sure if it has been ruled out.