The overly simplified explanation I'm giving myself right now is dark energy causes the opposite of what gravity does, that's why the universe is expanding. Now where gravity is a force, why dark energy is "energy"? Why it's not called "Dark Force" instead? I think I must be missing something here.
Answer
Because as far as we understand general relativity, it's not doing "the opposite of what gravity does." Gravity can be locally attractive or repulsive, depending on whether the stress-energy content satisfies or violates the strong energy condition. For ordinary matter, the stress-energy is dominated by the mass, the SEC holds, and its gravity is attractive. But this needn't be the case in general.
Since gravity depends on more than the (mass-)energy content, in cosmological models, the universe accelerates or decelerates proportionally to $\rho+3p$, where $\rho$ is the energy density and $p$ is the pressure. The factor of $3$ for the pressure comes from the fact that there are three spatial dimensions, but only one temporal dimension. In particular, the cosmological constant corresponds to the case of a perfect fluid with $p = -\rho$ (which can be generalized to other "equations of state"), and so a positive "dark energy" density has a repulsive effect, not because it's "the opposite of what gravity does", but rather because its negative pressure gravitates in addition to its energy.
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