The existence of dark matter is primarily inferred from its gravitational effects on visible matter in the universe, such as galaxies and galaxy clusters. If there were no dark matter in our universe, we would expect to see several observable consequences:
Galaxy Rotation Curves: Dark matter is invoked to explain the observed rotation curves of galaxies. Without dark matter, the velocity of stars in galaxies would decrease as we move away from the galactic center. However, observations show that the velocities remain relatively constant or even increase, suggesting the presence of unseen mass in the form of dark matter.
Galaxy Cluster Dynamics: Dark matter is crucial in explaining the dynamics of galaxy clusters. Without dark matter, clusters would not have enough mass to keep galaxies bound within them as they move through space. The observed motions of galaxies within clusters indicate the presence of additional matter, which is likely dark matter.
Gravitational Lensing: Dark matter affects the path of light traveling through space, causing gravitational lensing. This phenomenon can be observed when the light from distant galaxies is bent by the gravitational field of foreground objects. Gravitational lensing provides further evidence for the existence of dark matter.
Large-Scale Structure Formation: Dark matter plays a significant role in the formation of large-scale structures in the universe, such as filaments and galaxy superclusters. Without the gravitational influence of dark matter, the observed distribution of galaxies and the formation of cosmic structures would be difficult to explain.
If none of these observable phenomena were detected, it would strongly suggest that our understanding of the universe's mass distribution is incomplete, and the concept of dark matter would need to be revisited or revised. However, it's important to note that the cumulative evidence from various independent observations strongly supports the existence of dark matter, making it a well-established concept in modern astrophysics.