Billion and billion years ago, this space, is expanding from a pinprick size until today, a space where we call it Universe. Since space starts to expand, the energy it possesses for expanding purpose is strong enough to push away the border of the Universe. Initially, there is nothing in pinprick size space. Once it expands, it starts generating energy to form stars.
What is galaxy actually? It is a plate-like island which consists of thousands and thousands stars in it. Start from forming the stars, and the stars are grouped to be stars cluster(either open or globular cluster), and clusters are grouped to become galaxy. Furthermore, galaxies are grouped again to become today’s Universe. Similarly, in this Universe, there are thousands and thousands of galaxies consisted of. Our solar system, which consists of eight planets, is merely one of the dots in the galaxy that may be neglected by comparing to the others more tremendous objects.
Try to imagine, how small are we and how huge is the Universe. Galaxy is only an island in Universe, where there are many shapes of it. Some galaxies look spiral shape, we call it Spiral galaxy, like Milky Way, where our solar system is located. Besides, there are others look ellipse. We call it elliptical galaxy. This can be evidenced by the nearest galaxy with ours, about 4.3 light years away, Andromeda.
Each galaxy has a center where a black hole is located. As we know, black hole is a super-massive object that nothing can escape from it, not even the light. So, what do you know if a black hole is displaced at the center of galaxy? Black hole has robust gravity which can attract the objects around it towards itself. So, as black hole is at the middle of the galaxy, and the objects around it, such as stars, nebulae, planetary systems, and other else, will be attracted by it. However, the attraction force is not directly towards it. Just like our solar system, the planets are orbiting it but not directly move towards it although the Sun has robust gravitational force. So as the stars around the black hole in the galaxy, they are orbiting around the black hole and moving nearer and nearer to the center of it. Finally, the end of world. But, this event is not impossible to occur in this era because our solar system are still faraway and may need more than thousands and thousands years again to come closer to the center.
Laconically, galaxy can be imagined as the island in the universe. And the universe can be imagined like the ocean.