The nature of dark matter is still a subject of ongoing research and investigation. While the leading hypothesis suggests that dark matter consists of an, as yet undiscovered, subatomic particle, there are alternative theories and speculative ideas that have been proposed. Here are a few alternative explanations that have been considered:
Modified Gravity: Instead of positing the existence of new particles, some scientists have proposed modifications to the laws of gravity. These modifications would alter our understanding of gravity at larger scales, such as galactic and cosmological levels, and explain the observed phenomena without invoking dark matter. Examples of modified gravity theories include MOND (Modified Newtonian Dynamics) and TeVeS (Tensor-Vector-Scalar gravity).
Primordial Black Holes: It is conceivable that dark matter consists of primordial black holes, which are hypothesized to have formed in the early universe shortly after the Big Bang. These black holes would be smaller and less massive than those formed by collapsing stars, and they could account for the gravitational effects attributed to dark matter.
Macroscopic Dark Matter: Another possibility is that dark matter is composed of macroscopic objects rather than subatomic particles. These objects, such as large clouds of cold gas or massive compact halo objects (MACHOs), could exist in various forms, including planets, brown dwarfs, or other astrophysical bodies that emit little or no light.
Extra Dimensions: Some theories that extend beyond the standard model of particle physics propose the existence of extra dimensions. In these scenarios, dark matter could be localized or have properties related to these additional dimensions, leading to its weak interaction with ordinary matter and the gravitational effects we observe.
It's worth noting that while these alternative explanations have been proposed, they often face challenges in fully accounting for all the observed phenomena associated with dark matter. The particle-based hypothesis remains the most widely explored and supported by a range of experimental and observational data. Nonetheless, ongoing research aims to shed more light on the nature of dark matter and determine which, if any, of these alternative explanations may hold true.