The Universe has a long history, spanning nearly 14 billion years. Over that time, a relatively uniform soup of hot sub-atomic particles has been transformed into a stunningly diverse landscape, filled with complex and beautiful objects, like the Andromeda Galaxy you see above. The Universe today is populated by billions of galaxies with a wide range of colors, shapes, and sizes. No two galaxies are alike, just as no two people on Earth are exactly the same, but there are still many unanswered questions about why that is and how present-day galaxies ended up the way they are.
Astronomers have only been intentionally studying the Universe outside our own galaxy, the Milky Way, since the 1920s. At that time, Edwin Hubble and others began measuring the distances to the curious "nebulae" they saw in images (spoiler alert: the Andromeda Galaxy plays an important role in that story too!). Now, less than a hundred years later, the study of galaxies outside the Milky Way---what we call "extragalactic astronomy"---is one of the largest sub-fields in modern astrophysics. Owing to dramatic advances in engineering and technology over the last few decades, we can use the combined power of giant telescopes and supercomputers to help us understand how galaxies grow and change over time.
One of the challenges inherent to observational astronomy is that we can only observe a given galaxy at one point in time, even though we want to know what it was like throughout its life. It's like trying to study human development when you only have snapshots of different people taken decades apart! To get around this challenge, I use some of the largest telescopes in the world to learn about the internal properties of distant galaxies, including those that were forming more than 10 billion years ago, when the Universe was still very young. Using detailed spectroscopic observations of individual galaxies, we can look for evidence of what those galaxies were doing in the past. We can also use knowledge of their chemical abundance patterns (how much oxygen, carbon, etc. is in a galaxy) to link them to galaxies that have the same "cosmic DNA", but that we observe at different points in the past. Combining what we know about galaxies throughout the Universe's history enables us to understand what the life of a typical galaxy might have looked like.
Now is an exciting time to be working on these problems, because we are building new telescopes that will let us peer back to the dawn of cosmic history and allow us to study galaxies everywhere in much greater detail than is currently possible. These facilities---like the James Webb Space Telescope, The Nancy Grace Roman Space Telescope, and the Giant Magellan Telescope---will help answer many of our outstanding questions about galaxies. They will also undoubtedly reveal new frontiers for current and future astronomers to explore over the next century of extragalactic astronomy.