In the standard cosmological model , the Universe consists mainly of two invisible substances : vacuum energy with constant mass-density \rho _ { v } = \Lambda / ( 8 \pi G ) ( where \Lambda is a ‘ cosmological constant ’ originally proposed by Einstein and G is Newton ’ s gravitational constant ) and cold dark matter ( CDM ) with mass density that is currently \rho _ { DM, 0 } \sim 0.3 \rho _ { v } . This ‘ \Lambda CDM ’ model has the virtue of simplicity , enabling straightforward calculation of the formation and evolution of cosmic structure against the backdrop of cosmic expansion . Here we review apparent discrepancies with observations on small galactic scales , which \Lambda CDM must attribute to complexity in the baryon physics of galaxy formation . Yet galaxies exhibit structural scaling relations that evoke simplicity , presenting a clear challenge for formation models . In particular , tracers of gravitational potentials dominated by dark matter show a correlation between orbital size , R , and velocity , V , that can be expressed most simply as a characteristic acceleration , a _ { DM } \sim 1 km ^ { 2 } s ^ { -2 } pc ^ { -1 } \approx 3 \times 10 ^ { -9 } cm s ^ { -2 } \approx 0.2 c \sqrt { G \rho _ { v } } , perhaps motivating efforts to find a link between localized and global manifestations of the Universe ’ s dark components . ark matter , dark energy , \Lambda CDM