  Interstellar dust is a major foreground contaminant for many observations and
a key component in the chemistry of the interstellar medium, yet its properties
remain highly uncertain. Using low-resolution spectra, we accurately measure
the extinction curve - a diagnostic of the grain properties - for 130 million
stars, orders of magnitude more than previously available, allowing us to map
its variation in the Milky Way and Magellanic Clouds in 3D in unprecedented
detail. We find evidence that accretion is the dominant mechanism of grain
growth in moderately dense regions, with coagulation dominating at higher
densities. Moreover, we find that the extinction curve flattens in star-forming
regions, possibly caused by cycling of large grains formed in molecular clouds,
or by preferential destruction of small grains by supernova shocks.
