The dark energy idea is just one component in a recent major overhaul in cosmological theory, based on observations that contradict prior theories. It turns out that only 4% of mass-energy is the conventional kind we see around us. Fully 74% of all mass-energy is now thought to be dark energy, while 22% is believed to be "dark matter", a category of matter that has been hypothesized to account for the observed, anomalous orbital behavior of galaxies.

Dark energy has the name it does because it doesn't interact with ordinary matter except as a weak repulsive force that is only apparent at great ranges where gravitation can be overwhelmed. Dark matter has the name it does for much the same reason — it interacts gravitationally with ordinary matter, but it doesn't have any other known properties or interactions, and efforts to detect it have failed.

If the present model for dark energy is correct, that is, if it is a constant repulsive force that doesn't decline in step with the mass-energy density of the universe, this would represent a very serious challenge to a basic physical principle: conservation of energy. But the measured profile of galaxy motions seems to support this constant-repulsion idea.

Some quantum theories argue for a much higher value for cosmological repulsion, one that is certainly not observed, so theorists have been searching for a mechanism that balances two opposing forces and produces the resulting force as a residual outcome.

When dark energy was first proposed to explain the velocity profiles of galaxies over time, some researchers attempted to associate dark energy with so-called "vacuum energy" or "zero-point energy." Vacuum energy results from a quantum fluctuation of the vacuum, a process that creates virtual, short-lived particle pairs. These virtual particles are not supposed to become real particles or to yield any energy, for if they did this would upset the energy bookkeeping of the universe (just as dark energy seems to).

Stephen Hawking has proposed virtual particles as a way for black holes to eventually evaporate. This idea relies on the idea that one of a pair of virtual particles would fall through the event horizon of a black hole, while the other of the pair, now deprived of its partner, would escape into ordinary space. This idea doesn't violate conservation of energy because the net energy of the freed particle is taken from the black hole, a process that drains away the mass-energy of the black hole, which eventually reverts to ordinary matter in a huge explosion.

I mention this because it shows that virtual particles, ordinarily thought to be a pure theoretical construct with no physical effects, might actually play a part in black hole cosmology and possibly other areas. Nevertheless, for various reasons it seems that dark energy, whatever it turns out to be, cannot rely on an explanation that involves virtual particles or vacuum energy.

It is relatively easy to write a dark energy mathematical treatment that mimics reality, but this doesn't imply that dark energy is understood. Science that can only describe has always been considered a weak substitute for science that explains.

But enough theorizing. In the next and last page of this article, I provide a Java applet that interactively models dark energy using the mathematical methods presented earlier. This will provide the reader with a hands-on, intuitive sense of how dark energy behaves in a realistic orbital model.