  Magnetic-domain structure and dynamics play an important role in
understanding and controlling the magnetic properties of two-dimensional
magnets, which are of interest to both fundamental studies and
applications[1-5]. However, the probe methods based on the spin-dependent
optical permeability[1,2,6] and electrical conductivity[7-10] can neither
provide quantitative information of the magnetization nor achieve nanoscale
spatial resolution. These capabilities are essential to image and understand
the rich properties of magnetic domains. Here, we employ cryogenic scanning
magnetometry using a single-electron spin of a nitrogen-vacancy center in a
diamond probe to unambiguously prove the existence of magnetic domains and
study their dynamics in atomically thin CrBr$_3$. The high spatial resolution
of this technique enables imaging of magnetic domains and allows to resolve
domain walls pinned by defects. By controlling the magnetic domain evolution as
a function of magnetic field, we find that the pinning effect is a dominant
coercivity mechanism with a saturation magnetization of about 26~$\mu_B$/nm$^2$
for bilayer CrBr$_3$. The magnetic-domain structure and pinning-effect
dominated domain reversal process are verified by micromagnetic simulation. Our
work highlights scanning nitrogen-vacancy center magnetometry as a quantitative
probe to explore two-dimensional magnetism at the nanoscale.
