  Early expansion plays a fundamental role in the dynamical evolution of young
star clusters. However, until very recently most of our understanding of
cluster expansion was based only on indirect evidence or on statistically
limited samples of clusters. Here we present a comprehensive kinematic analysis
of virtually all known young ($t<300$ Myr) Galactic clusters based on the
improved astrometric quality of the Gaia DR3 data. Such a large sample provides
the unprecedented opportunity to robustly constrain the fraction of clusters
and the timescale during which expansion has a prominent impact on the overall
kinematics. We find that a remarkable fraction (up to $80\%$) of clusters
younger than $\sim30$ Myr is currently experiencing significant expansion,
whereas older systems are mostly compatible with equilibrium configurations. We
observe a trend where the expansion speed increases with the clustercentric
distance, suggesting that clusters undergoing expansion will likely lose a
fraction of their present-day mass. Also, most young expanding clusters show
large sizes, possibly due to the expansion itself. A comparison with a set of
N-body simulations of young star clusters shows that the observed expansion
pattern is in general qualitative agreement with that found for systems
undergoing violent relaxation and evolving toward a final virial equilibrium
state. However, we also note that additional processes likely associated with
residual gas expulsion and mass loss due to stellar evolution are also likely
to play a key role in driving the observed expansion.
