Could Dark Energy just be particles with negative mass?
If dark energy would consist of particles, it would dilute with the growing radius of the universe to the third power, since the total number of particles would stay the same while the volume increases. What observations found was that dark energy rather behaves like a constant which does not thin out, that's why it is also known as the cosmological constant. That means even if the universe expands, the amount of dark energy per cubic meter stays (at least approximately) the same.
Matter and dark matter are also evenly distributed throughout the observable universe, at least on the largest scales. What makes dark energy different isn't that it is uniformly distributed, but that it has a constant density. The amount of dark energy per cubic meter of universe is the same regardless of the total volume of the universe. If the universe is twice as big, there is twice as much dark energy, so the universe expands twice as fast. Hence the accelerated expansion. This must be a property of space itself. If these were particles they would simply dilute away as volume increased.
“If dark energy would consist of particles, it would dilute with the growing radius of the universe to the third power, since the total number of particles would stay the same while the volume increases. What observations found was that dark energy rather behaves like a constant which does not thin out, that's why it is also known as the cosmological constant. That means even if the universe expands, the amount of dark energy per cubic meter stays (at least approximately) the same.”
The answer above was conventionally correct. Modern science has now developed further since then. There was a result last year by an Oxford professor Jamie Farnes, widely discussed in international media, about possible creation of negative mass particles. http://arxiv.org/abs/1712.07962 When negative mass particles are created by a creation field, they do not thin out and can behave as a cosmological constant. Apparently the idea is also testable.