The first topic you treat in freshman physics is showing how a ball shot straight up out of the mouth of a cannon will reach a maximum height and then fall back to Earth, unless its initial velocity, known now as escape velocity, is great enough that it breaks out of the Earth' gravitational field. If that is the case, its final velocity is however always less than its initial one. Calculating escape velocity may not be very relevant for cannon balls, but certainly is for rocket ships.
The situation with the explosion we call the Big Bang is obviously more complicated, but really not that different, or so I thought. The standard picture said that there was an initial explosion, space began to expand and galaxies moved away from one another. The density of matter in the Universe determined whether the Big Bang would eventually be followed by a Big Crunch or whether the celestial objects would continue to move away from one another with decreasing acceleration. In other words one could calculate the Universe's escape velocity. Admittedly the discovery of Dark Matter, an unknown quantity seemingly five times as abundant as known matter, seriously altered the framework but not in a fundamental way since Dark Matter was after all still matter, even if its identity is unknown.
This picture changed in 1998 with the announcement by two teams, working independently, that the rate of acceleration of the Universe's expansion was increasing, not decreasing. It was as if freshman physics' cannonball miraculously moved faster and faster as it left the Earth. There was no possibility of a Big Crunch, in which the Universe would collapse back on itself. The groups' analyses, based on observing distant stars of known luminosity, supernovae 1a, was solid. Sciencemagazine dubbed it 1998's Discovery of The Year.
The cause of this apparent gravitational repulsion is not known. Called Dark Energy to distinguish it from Dark Matter, it appears to be the dominant force in the Universe's expansion, roughly three times as abundant as its Dark matter counterpart. The prime candidate for its identity is the so-called Cosmological Constant, a term first introduced into the cosmic gravitation equations by Einstein to neutralize expansion, but done away with by him when Hubble reported that the Universe was in fact expanding.
Finding a theory that will successfully calculate the magnitude of this cosmological constant, assuming this is the cause of the accelerating expansion, is perhaps the outstanding problem in the conjoined areas of cosmology and elementary particle physics. Despite many attempts, success does not seem to be in sight. If the cosmological constant is not the answer, an alternate explanation of the Dark Energy would be equally exciting.
Furthermore the apparent present equality, to within a factor of three, of matter density and the cosmological constant has raised a series of important questions. Since matter density decreases rapidly as the Universe expands (matter per volume decreases as volume increases) and the cosmological constant does not, we seem to be living in that privileged moment of the Universe's history when the two factors are roughly equal. Is this simply an accident? Will the distant future really be one in which, with Dark Energy increasingly important, celestial objects have moved so far apart so quickly as to fade from sight?
The discovery of Dark Energy has radically changed our view of the Universe. Future, keenly awaited findings, such as the identities of Dark Matter and Dark Energy will do so again.