First, that fellow "forgottenmorals" has mistakenly responded to "A"s answer with a bunch of info about dark matter, although both this question and A's answer were about dark energy. These two things are not the same (although, in forgottenmorals' defense, one of the few things we know about dark energy is that it interacts very weakly with ordinary matter, which is the main detail they are discussing).
Moreover, A's answer is actually correct… well… at least that is one of the possibilities that is probably the most popular, i.e. that dark energy is the what is called the "zero point energy," which is the energy the vacuum (i.e. ground state -- the state of lowest possible energy). Even though we colloquially (read: naively) describe the vacuum state as a state with nothing in it, if the quantum field theories known as quantum electrodynamics (QED, the quantum theory of electricity and magnetism), quantum chromodynamics (QCD, the quantum theory of the strong interaction that binds quarks together to form protons, neutrons, and the other hadrons), and quantum flavordynamics (QFD, the quantum theory of the weak interaction, which is how fermions interact with each other) are correct (or a combination of them, like electroweak theory [EWT]), then the vacuum is brimming with particle-antiparticle creations/annihilations, and these events produce energy, so the state of lowest possible energy does not contain zero energy, as one might expect. I should point out that one should not let my use of "if blahblahblah are correct" lead them to believe that these are highly uncertain, speculative theories: they have been experimentally verified to astounding accuracy (look up the anomalous magnetic moment of the electron). In the energy regimes we are able to probe experimentally, these theories are remarkably accurate models of reality, so they can certainly be trusted quite well. Without knowing, however, if quantum field theory is the correct formalism for the "most fundamental" (whatever that might mean) description of the universe, we can't say that these expectations will certainly hold, but we definitely seem to be on the right track -- we can measure the aforementioned vacuum energy.
That being said, it is still unclear whether or not this is the correct explanation for dark energy. Essentially, in order to write out explicitly what needs to be shown in order for the idea A is describing to be demonstrated is that the vacuum energy described above is what is called the cosmological constant, and that the cosmological constant is what we call dark energy. The second of these identifications is something most people expect to be true, and so I won't say too much about it, but the first identification is not without its hurdles. The theoretical prediction of how much vacuum energy the above creation/annihilation processes would generate is on the order of 1 planck mass, but (according to the most recent results form the Planck mission) the measured cosmological constant is of the order 10^(-122) planck mass, so, as you can see, there is an ENORMOUS discrepancy between the two. In order to identify them, one needs to kill off most of the vacuum energy our theories seem to predict. One of the main motivating reasons for the introduction of supersymmetry (that you've likely heard of) is that it would solve this problem. If our universe was perfectly supersymmetric, all the vacuum energy from fermions (particles like electrons, quarks, and so on) would exactly cancel the vacuum energy from the bosons (photons, Higgs, etc.). Then the tiny, tiny, tiny value we measure for the cosmological constant can be explained away as some slight curvature inherent to spacetime. The problem is that our universe is emphatically not supersymmetric (we haven't observed supersymmetry at all thus far, so in the everyday length scale of our ordinary lives, supersymmetry definitely isn't a thing), and so this symmetry would only hold on certain scales. This implies that there would not be as much of a cancellation, and it turns out that, *if* supersymmetry is a thing and is broken at a scale slightly above where we know it doesn't hold (i.e. slightly above what they're probing at the LHC), then is would only cancel about 10^(-60) of the aforementioned 10^(-122), i.e. only about half. This leaves us with almost as big of a problem reconciling the measured value of the cosmological constant with the vacuum energy as we originally had, and so something else would have to do the rest of the work. I am unaware of anything that has been proposed to compensate for the rest, so if the vacuum energy is the cosmological constant (which, remember, would be needed for A's explanation of dark energy to be correct), then we've still got a lot of work to do to figure out why.
(By the way, for any budding physicists out there, an excellent review article on the pre-dark energy cosmological constant problem is Weinberg's classic 1989 paper:
http://www.itp.kit.edu/~schreck/general_relativity_seminar/The_cosmological_constant_problem.pdf
For the post-dark energy CCP, Sean Carroll's 2000 review article is pretty good (although it could be updated a bit):
http://arxiv.org/abs/astro-ph/0004075)
Anywho, apart from that explanation of dark energy, some of the other contenders are:
-- Quintessence scalar fields (difficult to describe to non-physicists, but the idea is that the dark energy density is dynamical, i.e. changes over time, unlike the cosmological constant explanation, where the density remains the same);
-- Our prevalent cosmological models mostly assume that the universe is homogeneous, but some people propose inhomogeneity of the universe as an explanation for the accelerated expansion, i.e. there isn't actually any dark energy;
-- Some propose that our theory of gravity is no longer valid on large scales, and needs to be modified (in essence, the idea is that the force of gravity might not be proportional to 1/r^2, as in Newtonian gravity, but something different like 1/r^2+1/r^6. Since 1/r^6 is so much smaller than 1/r^2 for short distances, the contribution of the second term would be negligible, so we would only notice it on large scales);
-- Recently I went to a talk by Kerson Huang out of MIT describing how the existence of the Higgs field implies that the universe is a superfluid on large scales, and some well-known properties of superfluids (e.g. quantum vortices) can explain dark energy (as well as dark matter, and his model provides a mechanisms for the Big Bang, for inflation, and for the creation of matter around the time of the Big Bang, so it definitely seems interesting, but we'll have to wait and see how it looks after the details are worked out further, and it is tested against the data).
One final note about all of this, and why we expect the cosmological constant to be the right answer. We think of the universe as a perfect fluid, and for such fluids there is something called the equation of state: p=wρ, where p is the pressure of the fluid, ρ is the energy density, and w is called the "order parameter." It turns out that if dark energy is just the cosmological constant, then w=-1. The most up-to-date measurements of the order parameter give a value of -1.04 +/- 0.7. Quintessence models all involve order parameters either smaller than -1 or between -1/3 and -1, so it is looking like the cosmological constant model is probably the answer.
Lastly, about the origin of life on Earth… that is an entirely separate issue. As Donut Tim said, it is important not to confuse this issue with biological evolution, which is about the emergence of diversity of life, not the emergence of life itself. The leading contender for that question is abiogenesis, but we're still not entirely sure. I'm afraid you'll have to look up more on abiogenesis yourself, as this is not my area of expertise.
EDIT: I should also point out that, as I mentioned above, the only real mechanism we have to canceling out some unwanted vacuum energy (unwanted for anyone who wants to identify it with the cosmological constant/dark energy) is provided by supersymmetry, and this has been facing some hardships lately. Although not ruled out, there have been some serious constraints put on it. In fact, just recently, a collaboration between Harvard, Yale, UCLA, and York University (I think a university in Austria is also involved, but don't recall the name at the moment) called ACME (Advanced Cold Molecule Electron….. dipole moment…. guess a reference to some cartoons meant so much for the last tw0 words ;) ) measured how perfectly spherical the electrons in thorium atoms are. The idea is that if there are particles we haven't observed yet because high energy is needed to find them, then they would induce something called a dipole moment in the electron, which would warp its shape a little, so it wouldn't be spherical anymore. Their results indicate that it is still perfectly spherical at the scales they can currently probe, which means no dipole moment in that scale, which means (likely) no new particles in that scale. One consequence f this is that, according to the ACME team, if supersymmetry is part of the universe, the extra particles it predicts must lie outside of the energy regime that the LHC is capable of probing, i.e. we are very unlikely to find any sign of supersymmetry with the LHC…. and so, if supersymmetry exists, we won't find out for quite a while. Also, many models involving SUSY predict particles that would be detectable at the LHC, so these ACME results are basically saying that those models can pretty much be ruled out. Just something to keep in mind.