Caption: "The color and shape of a galaxy is largely controlled by gas (usually ∼ 72--75 % hydrogen (He) 25 % helium (He), 0 to 2--3 % metals by mass fraction) flowing through an extended galactic halo around it. All modern simulations of galaxy formation and evolution find that they CANNOT explain the observed properties of galaxies without modeling the complex accretion and "feedback" processes by which galaxies acquire gas (usually of primordial cosmic composition maybe sometimes somewhat enriched by metals from stellar nucleosynthesis and supernova nucleosynthesis from past outflows from galaxies) and then later expel it after stellar nucleosynthesis and supernova nucleosynthesis. Hubble Space Telescope (HST) spectroscopic observations show that galaxies like the Milky Way recycle gas while starburst galaxies will lose gas to intergalactic space and become "red and dead" (i.e., quenched galaxies)." (Somewhat edited.)

Credit/Permission: NASA, ESA, A. Feild, STScI 2012 / Public domain.
Image link: NASA's Hubble Confirms That Galaxies Are the Ultimate Recyclers. See also NASA: A Tale of Two Galaxies. Local file: local link: metallicity_evolution.html.
File: Cosmology file: gas_inflow_outflow.html.