If there is a black hole in the center of a galaxy, then it will probably suck the galaxy into it eventually, but I don't know if every galaxy has to have black hole in its center, or if it's not possible for a body to have stable orbit around such black hole.
Dark matter and dark energy are not necessarily actual matter and energy. These are just placeholder names for phenomena that we don't yet understand that look *like* it were matter and energy at work, that we can't see. It could turn out that it's not an additional matter, but that our equations were wrong, or that we were looking in wrong place/wrong way.