At the risk of oversimplifying, this is the lack of noticeably non-random fluctuations in the cosmic microwave background spectrum measured by Planck.

FORBES: Cosmic Inflation Model Gets A Boost From Planck Data

Because radiation from the Crab nebula is so energetic, the electrons that created it must have been travelling very close to the speed of light so close that the difference between the two speeds would bump up against Planck-scale graininess, and show up in the spectrum of the radiation.

ECONOMIST: Planck-scale physics