The vast distances between galaxies in the universe are indeed measured in light-years, which is a unit of distance representing the distance that light travels in one year. Light travels at an incredibly high speed of about 299,792 kilometers per second (186,282 miles per second). However, even with this immense speed, the vastness of the universe means that light takes a significant amount of time to travel from distant galaxies to reach us.
When we observe a galaxy that is, for example, one million light-years away, we are essentially seeing that galaxy as it appeared one million years ago because the light we receive today left the galaxy one million years ago. In a sense, we are looking into the past.
The light emitted by galaxies, stars, and other celestial objects travels through space until it reaches our telescopes or our eyes. As it travels, it moves at a finite speed, and the distances involved can be so immense that it takes a long time for the light to reach us. However, once the light does reach us, we can observe and study it.
It's important to note that our understanding of distant galaxies relies on the assumption that the laws of physics remain consistent across vast distances and long periods of time. This assumption has been supported by extensive observations and experimental evidence. By studying the light we receive from distant galaxies, astronomers can learn about the universe's history, its expansion, the formation of galaxies, and other astrophysical phenomena.
In summary, while galaxies are located at tremendous distances from us, the light they emit eventually reaches us, allowing us to observe and study them, albeit seeing them as they appeared in the distant past.