The Big Bang model is supported by strong evidence that the observable universe began hot and dense and has since expanded and cooled, though questions linger about the very earliest moments and the missing physics.

Alternative cosmologies—bouncing, cyclic, modified gravity, and quantum-origin models—exist but have not yet matched the standard model across all observations.

An outstanding issue is the lithium-7 abundance discrepancy, an unresolved mismatch in Big Bang nucleosynthesis predictions.

Dark matter remains essential to the model, yet its identity is still unknown despite decades of experimental searches.

The observable universe may be only a part of a larger cosmos with unknown global topology and size, leaving the full shape of the universe unresolved.

The Hubble tension shows a persistent gap between expansion rates measured locally and those inferred from the cosmic microwave background, hinting at potential new physics.

Inflation helps explain homogeneity, flatness, and the seeds of structure, but its underlying mechanism remains unknown and no single model has been confirmed.

Lambda-CDM remains a successful framework, but current tensions and observations suggest future theories could extend or revise it rather than discard it.

The theory does not describe the absolute beginning; classical general relativity leads to a boundary with a singularity, underscoring the need for a quantum theory of gravity.

Baryon asymmetry—why matter survived over antimatter—remains unexplained, a fundamental issue for the universe’s composition.

Early galaxies and massive black holes observed by the James Webb Space Telescope challenge formation timelines and may require revisions to formation efficiency, not rejection of the Big Bang.