Dark energy is the name given to whatever is causing the expansion of the universe to accelerate. One theory predicts that an unchanging entity pervading space called the cosmological constant, originally suggested by Albert Einstein, is behind dark energy. But a popular alternative, called rolling scalar fields, suggests that whatever's causing dark energy isn't a constant, but has changed through time.
If that were true, though, it should have caused the values of other fundamental constants of nature to change, too. And a new measurement of one such constant, the ratio between the mass of the proton and mass of the electron, shows that this constant has remained remarkably steady over time.
The proton and the electron are two fundamental particles that make up the atoms inside stars, galaxies and people. Recently, a team of astronomers used the German Effelsberg 100-meter radio telescope to observe alcohol molecules in a distant galaxy, and found that the protons and electrons in those molecules' atoms weigh about the same as the ones right here on Earth.