Unsolved problems in astronomy

mysterious non-luminous matter (and/or radiation) comprising most of the matter in our observable universe

life that does not originate from Earth

repulsive property with constant energy density theorized to make up most of the universe’s energy content to account for its observed expansion

apparent contradiction between the lack of evidence and high probability estimates for the existence of extraterrestrial civilizations

plasma "atmosphere" of the Sun

1977 narrowband radio signal from SETI

cosmological contradiction between the observed darkness of the night sky and a static universe model

binary system in the constellation Cygnus

high-energy astrophysical phenomenon manifested as a transient radio pulse lasting only a few milliseconds

alternative explanation of the non-Newtonian rotation of galaxies

subject of cosmology

short-lived light, color, or change in appearance on the surface of the Moon

apparent mystery that the early Earth seems to have had liquid water, even though the young Sun was less bright, thus presumably completely freezing the Earth

star in the constellation Centaurus

A strangelet (pronounced ) is a hypothetical particle consisting of a bound state of roughly equal numbers of up, down, and strange quarks. An equivalent description is that a strangelet is a small fragment of strange matter, small enough to be considered a particle. The size of an object composed of strange matter could, theoretically, range from a few femtometers across (with the mass of a light nucleus) to arbitrarily large. Once the size becomes macroscopic (on the order of meters across), such an object is usually called a strange star. The term "strangelet" originates with Edward Farhi a

thumb|upright|Parkes Observatory that detected BLC-1

theoretical upper limit on the energy of cosmic ray protons

cosmic-ray particle with a kinetic energy greater than 1 EeV (10¹⁸ eV)

planet, or exoplanet or, perhaps on a somewhat smaller scale, a moon or exomoon, that has been disrupted, or destroyed, by a nearby, or passing, astronomical body or object

unexplained circular astronomical object detected only by radio waves

enigmatic features on the lunar surface

iPTF14hls is an unusual supernova star that erupted continuously for about 1,000 days beginning in September 2014 before becoming a remnant nebula. It had previously erupted in 1954. None of the theories nor proposed hypotheses fully explain all the aspects of the object.

star noted for unusual dimming events

Wikimedia list article

class of small, red galaxies

astronomical object

high-energy astrophysical process

absorption features in the spectra of astronomical objects in the Milky Way and other galaxies

Relationship between the stellar mass dispersion of a galaxy bulge and the mass of the supermassive black hole at its center

facet of general relativity

star in the constellation Scorpius

Photon field that fills our universe