Title speaks for itself.
Dark matter: We see extra attractive force, and we posit that there are particles which create such a force, and use the measure of that force to guess their locations.
Dark Energy: We see extra repulsive force.
Only thing is, dark energy is uniform. So I suppose the stuff would have to be (at least somewhat) uniformly distributed throughout the universe. How uniform do we know it to be? Could the "stuff" be somehow a part of empty space itself?
Answer
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.
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