Garden

The Garden is the last dome to be unlocked by Mario/Luigi in Super Mario Galaxy

During the end credits, It is shown that the Garden broke off the top of the Observatory with the spiral still on it. But when everything starts to be sucked down the black hole. The spiral breaks off the Garden (probably with the crown being flung off or still on the Garden) The Garden is not one of the last pieces of the Observatory to be sucked down the black hole and destroyed.