Quick Answer
If gravity universally repelled matter, the familiar structures created by gravitational attraction would be impossible to maintain. Planets would not naturally bind material together, stars could not form through gravitational collapse, and ordinary orbital systems would behave radically differently. The premise also conflicts with the way gravity is described in modern general relativity.
Gravity is the large-scale interaction that allows matter to assemble into stars, planets and galaxies. Reversing its sign would therefore attack the foundation of cosmic structure.
Earth would not stay together
If every piece of mass repelled every other piece, there would be no ordinary gravitational binding keeping the planet compact. Other forces act at microscopic scales, but they cannot simply replace gravity for a planet-sized body.
Stars could not form normally
Stars form when gravity pulls gas into dense regions until pressure and temperature become high enough for nuclear fusion. Reverse that attraction and the collapse process disappears.
Orbits change completely
Planetary orbits rely on an attractive central force. A repulsive force would push objects away rather than curve their paths into stable elliptical orbits in the usual way.
The universe would look different
Galaxies, clusters and large-scale structure depend on gravity. A universal repulsive interaction would require an entirely different mechanism to create the structures we observe.
SpeculativeThis is not a small modification to Newtonian gravity. It changes the sign of a fundamental interaction and would require rebuilding the physical theory from the ground up.
The bottom line
Repulsive gravity would not produce a slightly different Earth. It would prevent the gravitational assembly of the large structures that define the universe we know.
A note on our approach: Every article on WhatIfLab separates what current science establishes from what remains genuinely speculative. Where we cite a figure or finding, it reflects published, mainstream research at the time of writing — not a prediction dressed up as fact.