If the galaxy in your dream is dark colored, undetailed and huge, means that in the future you may face a problem and you will struggle against it. To dream of a bright colored, detailed and small galaxy foretells the problem which you can understand and control. The result of the problem will be positive. You should analyze every aspect of the problem or situation and find the most appropriate solution. You are capable to do so even if the entire galaxy against you in your dreams.
Alternatively, the galaxy in your dream may represent your creativity and imagination. Especially if in your dream you are excited by the galaxy and you are trying to scrutinize every side and detail of it.