Cepheid variable stars: variable starts with pulsation periods of 1 to 60 days whose period of variation is related to their luminosity.
Dark matter: nonluminous matter that is detected only by its gravitational influence
Elliptical galaxies: are round or elliptical, contain no visible gas and dust and lack hot, bright stars
Spiral Galaxies: contain a disk or spiral arms. Their halo stars are not Visible, but presumably all spiral galaxies have halos. Contain gas and Dust and hot bright O and B stars
Irregular galaxies: are a chaotic mix of gas, dust and stars with no obvious nuclear bulge of spiral arms
Local group: the small cluster of a few dozen galaxies that contains our milky way
Active galatic nucleus (AGN): the centers of active galaxies that are emitting large amount of excess energy
Olbers's paradox: the conflict between theory and evidence regarding the darkness of the night sky
Isotropy: the observation that, in its general properties, the universe looks the same in every direction
Homogeneity: the observation that, on the large scale, matter is uniformly spread throughout the universe
Dark energy: the energy believed to fill empty spaces and drive the acceleration of the expanding universe
the Elements-namely ordinary baryonic matter out of protons and neutrons as Well as are only a small part of the content of the universe. Cosmological obsersations say that 72% of the universe is dark energy, 23% is dark matter and 4.6% is visible baryonic matter which is stars, Planets and living beings. Dark matter has not yet been detected in a Particle physics detector and the nature of the dark energy is not yet Understood
Hydrogen and helium are 74% and 24% of all baryonic matter un the universe.