I've been reading many clever answers here about dark matter and dark energy that called my attention to this question. Since Einstein's theory relates matter and energy as different states of the same thing, is it valid to think about dark matter and dark energy in the same way? Are they two states of the same dark "thing"? Are they interchangeable?
The short answer to your question is that we don't know if dark matter and dark energy are manifestations of the same dark "thing". We know they both must exist to explain certain phenomena, but we still know very little about their make up so we cannot assume they are linked. For now, we think of them as separate, and we believe the cosmos to be composed of roughly 0.03% heavy elements (anything other than hydrogen and helium), 0.3% neutrinos, 0.5% stars, 4% free hydrogen and helium, 25% dark matter, and 70% dark energy. Here is how we define them separately:
Dark matter must exist to account for the gravity that holds galaxies together. If the only matter in the universe was matter we could directly detect, galaxies would not have had enough matter to have ever formed. The galaxies we observe today would fly apart because they wouldn't have enough matter to create a strong enough gravitational force to hold themselves together. Dark matter is also responsible for amplifying small fluctuations in the Cosmic Microwave Background back in the early universe to create the large scale structure we observe in the universe today.
Dark energy, which also goes by the names of the cosmological constant or quintessence, must exist due to the rate of expansion we observe for our universe. Not only is the universe expanding, but this expansion is also accelerating so the unknown 'anti-gravity' force at work is termed 'dark energy'.
Some researchers are searching for an explanation that encompasses both dark matter and dark energy. One example of such a theory uses a form of energy called a scalar field (it is a field because it has magnitude, energy and pressure, but it is scalar so it has no direction). Things would certainly be easier if we didn't need to have separate theories to explain dark matter and dark energy. However, other researchers look at dark matter and dark energy as two separate problems. For example, many string theories use supersymmetric particles to explain dark matter and make no connection to dark energy at all.