Biggest Mystery: What is Dark Energy?
When scientists are faced with a new mystery, they collect the available data and develop theories to explain the new data. Over time, new data will contradict some of those theories, and they will drop out of contentions. Others will be bolstered by the new data. This is the very process of science. Dark energy is no different from any other scientific mystery. Astronomers have theories for what it is, and develop new tests to differentiate those theories.
Astronomers have a few front-runners for what dark energy is, including the cosmological constant and quintessence, as discussed in this article. The primary difference between these two models is that the cosmological constant is a constant energy field filling all of space; whereas, quintessence allows for changes in the energy field over space and time.
Over time, new data will help astronomers determine which models work best. Some theories may require modification while others will be abandoned. We will all just have to wait and see.