Galaxy is a large collection of dust, gas and millions of stars, their solar systems and dark matter which are held together by gravity.
Some galaxies are smooth and oval shaped and some are spiral-shaped, the smooth and oval shaped galaxies are called as elliptical galaxies. There also other galaxies which are neither spiral nor oval, they have irregular shapes.
In our galaxy that is the milky way has a super-massive black hole in the middle.
Hence, galaxy includes all the options that is planets, stars, dust, interstellar gas, and dark matter.