If dark energy would consist of particles, it would dilute with the growing radius of the universe to the [third power][1], 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][2] which does not thin out, that's why it is also known as the [cosmological constant][3]. That means even if the universe expands, the amount of dark energy per cubic meter stays (at least approximately) the same.


  [1]: https://en.wikipedia.org/wiki/Friedmann_equations
  [2]: https://arxiv.org/abs/1303.5076
  [3]: https://www.google.at/search?q=cosmological%20constant