Where is the dark star in Mario and Luigi Bowser's inside story?

Answer 1

In the Final battle it is inside of Dark Bowser. After you battle with him for a while as Bowser, he should become bigger. When he does, breath fire on him and the dark star will fall out of his mouth. Then suck him into your stomach battle it as Mario and Luigi. You'll have to do this a few times before you finally defeat it.

Answer 2

When you get to it it's in the chest area with tons of enemies to fight!