ΛCDM appears to be extremely successful in predicting the cosmic microwave background and large-scale structure, including the observed distribution of galaxies both nearby and at high redshift. It has therefore become the standard cosmological framework within which to understand cosmological structure formation, and it continues to teach us about galaxy formation and evolution. For example, I used to think that galaxies are pretty smooth, that they generally grow in size as they evolve, and that they are a combination of disks and spheroids. But as I discussed in Section 3, HST observations combined with high-resolution hydrodynamic simulations are showing that most star-forming galaxies are very clumpy; that galaxies often undergo compaction, which reduces their radius and greatly increases their central density; and that most lower-mass galaxies are not spheroids or disks but are instead elongated when their centers are dominated by dark matter.
ΛCDM faces challenges on smaller scales. Although starbursts can rapidly drive gas out of the central regions of galaxies and thereby reduce the central dark matter density, it remains to be seen whether this and/or other baryonic physics can explain the observed rotation curves of the entire population of dwarf and low surface brightness (LSB) galaxies. If not, perhaps more complicated physics such as self-interacting dark matter may be needed. But standard ΛCDM appears to be successful in predicting the dark matter halo substructure that is now observed via gravitational lensing and stellar streams, and any alternative theory must do at least as well.
Acknowledgment: My research is supported by grants from NASA, and I am also grateful for access to NASA Advanced Supercomputing and to NERSC supercomputers.