TLDR: The more mass an object has, the stronger the gravitational force. A planet of this size cannot exist (or have the same density of a world like Earth) because the sheer gravity would had crunched all the matter together into a black hole.

Long Answer:

Mass is not equally distributed across all of space. To put it simply, there planets and stars, with lights years of space between them

Thus gravity. The result of the spacetime curvature caused by the unequal distribution of mass across the universe.

The more mass an object has, the stronger the gravitational force.

This hypothetical planet cannot have the same density as a rocky planet (like Earth) because such a HUGE ASS object (even if it somehow formed so) will probably have all its mass crunched together from the sheer gravitational force of it's own weight.

Realistically, this world should be portrayed as a weird black hole instead.