What is dark matter, and what percentage of the universe's total mass-energy content does it make up?

An unknown form of matter that does not emit or absorb light, comprising about 27% of the universe's total mass-energy The collective mass of all black holes in the universe, which makes them invisible to telescopes A type of cold gas cloud that blocks background starlight, making up about 27% of the universe Anti-matter left over from the Big Bang that fills the space between galaxies

What are the five independent lines of evidence that dark matter exists?

Galaxy rotation curves, gravitational lensing, galaxy cluster dynamics, the cosmic microwave background, and large-scale structure formation Direct particle detector results, neutrino oscillation data, pulsar timing, X-ray emissions, and supernova light curves Black hole shadow imaging, gravitational wave detection, quasar spectra, gamma-ray bursts, and stellar evolution models Hubble constant measurements, redshift surveys, dark energy surveys, cosmic ray data, and radio telescope maps

What is dark energy, and what observational evidence first confirmed its existence?

An unknown force that slows the expansion of the universe, confirmed by cosmic microwave background measurements An unknown form of energy causing the universe's expansion to accelerate, first confirmed by Type Ia supernova observations in 1998 The energy released by all stars in the universe, which collectively push galaxies apart through radiation pressure A quantum field produced by dark matter decay, confirmed by gravitational wave detectors in 2017

What is the Hubble tension, and why might it require new physics?

A disagreement between Hubble Space Telescope and James Webb Space Telescope measurements of distant galaxy distances The difficulty in measuring the exact distance between the Milky Way and the Andromeda galaxy A disagreement between two precise measurements of the universe's expansion rate — CMB measurements give about 67 km/s/Mpc while local distance ladder measurements give about 73 km/s/Mpc The observation that the Hubble constant appears to change over time as the universe ages

What is the black hole information paradox?

The mystery of why we cannot measure the mass of a black hole without entering its event horizon The conflict between quantum mechanics (which says information is never destroyed) and black hole physics (where information falling in seems to vanish when the black hole evaporates via Hawking radiation) The inability of current telescopes to image black holes directly because light cannot escape them The paradox that black holes appear to have infinite density at their singularities, violating known physics

What is the matter-antimatter asymmetry problem?

The observation that antimatter is heavier than matter, causing it to sink to the bottom of the universe The Big Bang should have created equal amounts of matter and antimatter, which would annihilate each other — yet the observable universe is made almost entirely of matter The difficulty in producing antimatter in particle accelerators because it requires more energy than matter The observation that antimatter travels backward in time, making it impossible to study in laboratories

What does the Lambda-CDM model describe, and what is its biggest shortcoming?

It describes the motion of stars and galaxies in the Milky Way and correctly predicts all observed stellar orbits It is the standard model of cosmology describing the universe's composition and evolution, but it cannot explain what dark energy (Lambda) or cold dark matter (CDM) actually are It describes how cold atomic gas forms stars, but cannot explain why galaxies have the masses they do It is a theory of everything that unifies gravity with quantum mechanics but has not been tested observationally

Why is the question of what happened before or at the Big Bang currently unanswerable?

The Big Bang happened so long ago that all evidence of what came before has been destroyed by entropy The concept of time has no meaning before the Big Bang, so the question is logically meaningless Our telescopes cannot see far enough back in time to observe the Big Bang directly Our current physics breaks down at the Big Bang singularity — a theory of quantum gravity is required to describe the extreme conditions of the very first moments

How is the James Webb Space Telescope helping answer questions about the first stars and galaxies?

JWST can detect gravitational waves from the first stars collapsing into black holes JWST can observe galaxies as they appeared only a few hundred million years after the Big Bang, revealing when the first stars formed and what the universe looked like during reionization JWST has directly imaged the cosmic microwave background, confirming the Big Bang theory with unprecedented precision JWST is searching for dark matter particles by detecting their faint X-ray emissions in the cores of galaxies

Why do many physicists consider the question of extraterrestrial life one of the most profound open questions in science?

The discovery of alien life would resolve the black hole information paradox and the Hubble tension simultaneously If life arose independently elsewhere, it would suggest life is a universal phenomenon driven by chemistry and physics, profoundly changing our understanding of biology, cosmology, and humanity's place in the universe Alien civilizations likely have already solved dark matter and dark energy, so contact would immediately resolve these problems Finding extraterrestrial life would prove that dark matter is actually composed of living organisms