  Rapidly growing black holes are surrounded by accretion disks that make them
the brightest objects in the Universe. Their brightness is known to be
variable, but the causes of this are not implied by simple disk models and
still debated. Due to the small size of accretion disks and their great
distance, there are no resolved images addressing the puzzle. In this work, we
study the dependence of their variability on luminosity, wavelength and
orbital/thermal timescale. We use over 5,000 of the most luminous such objects
with light curves of almost nightly cadence from $>5$ years of observations by
the NASA/ATLAS project, which provides 2 billion magnitude pairs for a
structure function analysis. When time is expressed in units of orbital or
thermal time scale in thin-disk models, we find a universal structure function,
independent of luminosity and wavelength, supporting the model of
magneto-rotational instabilities as a main cause. Over a $>1$~dex range in
time, the fractional variability amplitude follows $\log (A/A_0) \simeq 1/2
\times \log (\Delta t/t_{\rm th})$. Deviations from the universality may hold
clues on the structure and orientation of disks.
