Short answers, pulled from the story.
A galaxy is a system of stars, stellar remnants, interstellar gas, dust, and dark matter bound together by gravity. Most of the mass in a typical galaxy is dark matter, with only a few percent visible as stars and nebulae. Supermassive black holes are a common feature at the centers of galaxies.
Estimates range between 200 billion and 2 trillion galaxies in the observable universe. The Hubble Deep Field suggested about 125 billion, while a 2016 study led by Christopher Conselice estimated at least two trillion, a factor of ten more than appear directly in Hubble images.
Galaxies come in three main types: ellipticals, spirals, and irregulars, sorted by the Hubble classification system. Ellipticals are rated from E0, nearly spherical, to E7, highly elongated. The Milky Way is an example of a barred spiral galaxy.
Dark matter makes up most of the mass in a typical galaxy, and the galaxy rotation problem found by Vera Rubin in the 1970s is explained by it. A study of 27 Milky Way neighbors found a central mass of roughly 10 million solar masses in every dwarf galaxy, regardless of star count, suggesting galaxies are largely shaped by dark matter.
The Milky Way and the Andromeda Galaxy are moving toward each other at about 130 km/s and may collide in roughly five to six billion years, depending on their lateral movements. The Milky Way has collided and merged with other galaxies before, including one labeled the Kraken about 11 billion years ago.
A quasar is the most energetic and distant type of active galactic nucleus, powered by a supermassive black hole, and its luminosity can be 100 times that of the Milky Way. The nearest known quasar, Markarian 231, is about 581 million light-years from Earth, while UHZ1 lies roughly 13.2 billion light-years distant.