Dark Energy may be an illusion: Gravitons themselves may have mass

For what it's worth, physicists actually have a pretty good hunch for what dark energy might be. Given standard cosmology, dark energy would just be the cosmological constant Λ. This idea is encapsulated in the Lambda-CDM model of cosmology, which can be thought of as cosmology's counterpart to particle physics's Standard Model.

In fact, there's a very simple link between the Lambda-CDM model and the Standard Model. In Lambda-CDM, the cosmological constant is the energy density of space (ie, the energy density of the vacuum). According to the Lambda-QCD model, this energy density would create a negative pressure (per Einstein's field equations), which would effect the universe to expand, exactly as expected from dark energy.

In quantum field theory (QFT)/the Standard Model, the vacuum energy is not expected to be 0. Thus QFT offers a straightforward solution to the origin of the cosmological constant: it would simply be the vacuum energy of the quantum fields! Alas, there is a rather significant problem with this approach. In general the vacuum energy of QFT is practically infinite, whereas the cosmological constant is nearly 0. The only way, therefore, that we could have a near 0 cosmological constant is if the vacuums of the different quantum field theories described by the Standard Model nearly cancel-out. For this reason, the cosmological constant has been described as "the worst theoretical prediction in the history of physics".

I want every journalist who covers any scientific anything to pay into science education funds for every instance of the phrase "just a theory" that comes up in their writing. I don't care how much they pay. I just care that they pay.