  Despite rapidly growing disk observations, it remains a mystery what
primordial dust aggregates look like and what the physical and chemical
properties of their constituent grains (monomers) are in young planet-forming
disks. Confrontation of models with observations to answer this mystery has
been a notorious task because we have to abandon a commonly used assumption,
perfectly spherical grains, and take into account particles with complex
morphology. In this Letter, we present the first thorough comparison between
near-infrared scattered light of the young planet-forming disk around IM Lup
and the light-scattering properties of complex-shaped dust particles. The
availability of scattering observations at multiple wavelengths and over a
significant range of scattering angles allows for the first determination of
the monomer size, fractal dimension, and size of dust aggregates in a
planet-forming disk. We show that the observations are best explained by
fractal aggregates with a fractal dimension of 1.5 and a characteristic radius
larger than $\sim2~\mu$m. We also determined the radius of the monomer to be
$\sim200$ nm, and monomers much smaller than this size can be ruled out on the
premise that the fractal dimension is less than 2. Furthermore, dust
composition comprising amorphous carbon is found to be favorable to
simultaneously account for the faint scattered light and the flared disk
morphology. Our results support that planet formation begins with fractal
coagulation of sub-micron-sized grains. All the optical properties of complex
dust particles computed in this study are publicly available.
