In particle physics, the existence of a galaxy does not depend on whether it has been observed or not. The presence or absence of life in a galaxy is a separate matter and is not directly related to the field of particle physics.
From a cosmological perspective, galaxies are vast systems of stars, gas, dust, and other celestial objects held together by gravitational forces. They exist independently of our observations or knowledge of them. Our ability to observe galaxies is based on the detection of light or other electromagnetic radiation emitted by the objects within the galaxy. However, the existence of a galaxy itself is not contingent upon its observation.
When we observe distant galaxies, we are seeing them as they were in the past because the light from those galaxies takes time to reach us. This phenomenon is due to the finite speed of light. For example, if a galaxy is located 1 billion light-years away, the light we observe from it today actually left that galaxy 1 billion years ago. This does not mean that the galaxy did not exist before we observed it; it simply means that the light reaching us is from the galaxy's past.
In summary, the existence of a galaxy is not dependent on our observation, and its existence in the past does not change based on when we observe it. The study of particle physics focuses on the fundamental particles and forces that make up the universe, and it does not directly address the existence or non-existence of galaxies or life within them.