Imagine a basketball and a marble in space, very close to each other, and away from other forces... Would the?

You're absolutely correct. Every object with m exerts a force on every other, and that force is proportional to the mes of the objects and inversely proportional to the square of the distance between them. The gravitational force is extremely weak though, which is why you need large objects like planets to really see the effect, and why even the gravitational force of the entire earth can be overcome by a tiny magnet (or, you lifting something up...).