A looming mystery lies at the heart of modern cosmology, and it has everything to do with the ever-accelerating expansion of the universe. First discovered in 1998 by analyzing distant Type 1a supernovae using the Hubble Space Telescope (a discovery that eventually won the Nobel Prize in Physics in 2011), this perceivable expansion isn’t easily explained with any known matter or energy. So, the current front-running cosmological model—known as lambda cold dark matter, or ΛCDM—relies on dark energy to explain this accelerated expansion.