Galaxies and black holes, while they can persist for extremely long periods of time, are not necessarily "forever" entities. However, their lifetimes are significantly longer compared to human timescales. Let's consider each of them separately:
Galaxies: Galaxies are vast systems of stars, gas, dust, and dark matter held together by gravity. They come in various shapes and sizes, ranging from spiral galaxies like our Milky Way to elliptical and irregular galaxies. Galaxies can maintain their structure and existence for billions of years, but they are not static. They evolve and change over time due to various factors such as interactions and mergers with other galaxies, gravitational interactions, star formation, and the activity of the central supermassive black hole. Eventually, over very long timescales, galaxies may collide and merge, or they may disperse and fade away as new stars stop forming. So while galaxies can persist for immense durations, they are not everlasting.
Black holes: A black hole is a region in space where the gravitational pull is so strong that nothing, not even light, can escape from it. Black holes form when massive stars collapse under their own gravity, concentrating their mass in an incredibly small volume. They are characterized by their event horizon, a boundary beyond which nothing can escape. Black holes can grow in size by accreting matter from their surroundings or by merging with other black holes. However, black holes can also lose mass and eventually evaporate over extremely long timescales due to a phenomenon known as Hawking radiation. While this process takes an incredibly long time for stellar-mass black holes, it is still theoretically possible for them to gradually dissipate. Nonetheless, supermassive black holes, which reside at the centers of galaxies, are thought to have significantly longer lifetimes and may persist for much of the age of the universe.
In summary, while galaxies and black holes can exist for extraordinarily long periods, they are subject to various processes and interactions that can lead to changes and even eventual dissolution. However, the timescales involved are so vast that from a human perspective, they appear to be almost perpetual.