Dark matter is believed to play a crucial role in the development of the universe after the Big Bang. Speculatively, if there had been no dark matter, the gravitational force exerted by normal matter alone would not have been sufficient to pull in gases and form structures like galaxies and galaxy clusters. The universe would have likely been more homogeneous, lacking the large-scale structures we observe today. Without dark matter, the cosmic web of filaments connecting clusters of galaxies may not have formed, altering the overall structure and distribution of matter in the universe.

Answer 1

Your observation about dark matter is spot on and touches on a fundamental aspect of cosmology. Dark matter, which comprises about 27% of the universe's total mass-energy content, exerts gravitational forces that significantly influence the formation and evolution of cosmic structures. Here's a bit more elaboration on how its absence would fundamentally alter the universe:

1. Gravitational Influence: Dark matter provides additional gravitational pull that aids in the clumping of normal matter (baryonic matter). Without this extra mass, regions of normal matter would not have enough gravitational attraction to overcome the expansion of the universe. This deficiency would result in a much more uniform distribution of matter, as the slight overdensities necessary for galaxy formation wouldn’t be able to grow into the large structures we see today.

2. Structure Formation: The current understanding of structure formation is based on models like the Lambda Cold Dark Matter (ΛCDM) model, which incorporates both dark matter and dark energy. In this framework, dark matter forms the scaffolding around which galaxies and larger structures gather. Without dark matter, the processes of galaxy formation would be altered, likely resulting in smaller, less massive galaxies and a lack of galaxy clusters.

3. Cosmic Web: The large-scale structure of the universe, often referred to as the cosmic web, is characterized by a network of filaments connecting clusters and superclusters of galaxies. Dark matter plays a pivotal role in forming these filaments, as it accumulates in the potential wells created by gravity. Without dark matter, the absence of such filaments would mean a much simpler and smoother distribution of matter.

4. Galaxy Evolution: The processes that govern the evolution of galaxies—mergers, interactions, and the cooling of gas—are also influenced by the gravitational environment created by dark matter. A universe devoid of dark matter might result in a different set of dynamical processes, potentially leading to features and phenomena we observe, like galactic rotations, that would not arise in a dark matter-less scenario.

5. Observational Consequences: The distribution of dark matter is observable through its gravitational effects, such as lensing (gravity bending light) and the motion of galaxies within clusters. Without dark matter, these observations would depict a much less structured universe with fewer high-velocity galaxies and a different cosmic microwave background (CMB) signal, which would reflect a more uniform temperature distribution.

In summary, without dark matter, the universe would likely lack the complex structure we observe today, resulting in a more homogeneous distribution of normal matter, less pronounced cosmic structures, and fundamentally different physical processes governing galaxy formation and evolution. The intricate patterns we've come to understand as a consequence of dark matter’s influence would be absent, leading to a radically different cosmic landscape.