You may think physics has changed over the past 200 years, but it hasn't. Today, theoretical physicists can't understand why the universe is expanding at an observed rate that doesn't quite mesh with general relativity. Back in the 19th century, theoretical physicists couldn't understand how electromagnetic energy and gravity could propagate through empty space. The proposed solution in both cases was the same: there must some stuff out there that we can't see, yet affects the entire universe. In the past, that substance was aether. Is today's dark energy the modern equivalent?
Aether (also called ether) was a theoretical substance that supposedly permeated the entire universe, including solid matter, more or less evenly. While aether theories evolved over time, it was generally believed to be made of particles so tiny we couldn't detect them. The inherent properties of the aether determined many of the physical properties of the universe, such as the speed of light and the strength of gravity. These forces propagated as waves through the aether. Aether theory survived into the 20th century - Einstein even adapted it to fit his theory of special relativity, although it was so drastically changed that it was hardly aether theory at all. In his 1920 address "Ether and the Theory of Relativity," Einstein said:
The ether of the general theory of relativity is a medium which is itself devoid of all mechanical and kinematical qualities.
Dark energy is the theoretical source of the force that is causing the universe to expand at an accelerated rate. Physicists measure cosmic expansion by observing the redshift in the light from exploding stars. The rate of expansion they see doesn't fit into the equations of general relativity unless they add in what is basically an imaginary number, a force of some kind that causes the expansion. A form of energy that we are unable to observe directly and fills the universe more or less evenly (another flavor of the theory posits a scalar field of dark energy that would not be so homogeneous) creates this force. The energy may be an inherent property of space itself, sometimes known as vacuum energy, and it exerts a negative pressure. This negative pressure stretches space, causing a gravitational repulsion that makes the universe expand.
To be certain, neither theory is "bad science" in any way. They are the types of theories that physicists come up with when they are working out beyond the current observational abilities of humans. Eventually, physicists identified the dual wave/particle properties of electromagnetic energy. This, along with experiments that confirmed general relativity, negated the need for aether theory. Likewise, new experiments conducted with the Large Hadron Collider later this year could detect new particles like the Higgs boson that will give us additional clues to the physical makeup of the universe. Will they invalidate dark energy? Photo by: NASA.