5 is real (at the core) is a message recently said by the The Great Crown or Crownux (that as we know of)
What the message means is below.
What the message is is that there is 5 dimensions in the Mushroom Kingdom Wheel. And the 5th dimension is the Crown Dimension. In the Crown Dimension, Mario collected the rest of the D-stars (9 of them being replica) and got the 5 crown stars. Then Mario became the King of Dimensions!