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" ... It is for these reasons — for all of the properties of dark matter that we’ve been able to infer from astrophysical observations alone — that we conclude that dark matter is particle-like in nature. That doesn’t mean it can’t be a pressureless fluid, a type of clumpy dust, or that its cross-section is zero under every interaction except the gravitational one. What it does mean is that, if you try to replace dark matter with a field, that field must behave in a fashion that, from an astrophysical perspective, is not distinguishable from the behavior of a large set of massive particles. ... "
" ... The way we conventionally describe dark energy is through a single parameter: w, known as the equation of state. In physics, w relates the energy density of any form of energy to the pressure of that form of energy. For normal matter that moves at negligible speeds relative to the speed of light, w = 0, meaning that both normal matter and dark matter are pressureless. ... "