Mind-blowing discoveries and realizations that fundamentally change how we view the universe and to some extent even life itself in the late 20th century, off the top of my head:
* Cosmology. Most of the cosmological theories (e.g., Big Bang) didn't develop until after 1950, and the Hubble Space Telescope has advanced the field considerably.
* Supersymmetry/superstring theory. I hesitate to put this here because I think this is a wrong turn for theoretical physics, but it is a very well-known model that developed in the 1970s, with the superstring theory revolution occurring in the 1970s.
* Central dogma of molecular biology. Proposed in the late 1950s, this states that basically DNA -> RNA -> protein, and nothing goes back the other way. It has also taken a massive beating in recent decades, as we've discovered there's quite a few ways that information gets passed around in cells that don't follow this path (e.g., epigenetics, and it's an epic acerbic word match if you want to restrict epigenetic only to things like methylation of DNA or include any other heritable non-genetic traits as epigenetic).
* Plate tectonics. This became accepted geological theory only around the late 1960s.
This is just a list of pure science discoveries I can think of off the list of my head. Stretch to applied science advancements, and there's several that have caused massive revisions to fields--organic synthesis techniques, NMR spectroscopy, radiocarbon dating, genetic sequencing are all things that came to mind while compiling the above list. If you switch over to math/CS side of things, you can also consider that the development of RSA quite literally opened up a new world of what can be done with computers, or the Cook-Levin Theorem that really kicks off computability theory (and leads to the asking of the question P=NP). Pretty much everything about CS only develops in the late 20th and early 21st century.
Hell, that last comment prompted me to remember something else that rather fundamentally changes how we view the universe. It turns out that it's far easier to teach a computer how to play chess than speak with the intelligence of a three-year-old, never mind an adult. What can be more mind-blowing than discovering that we can't make a more effective learner than a toddler?