Where and when could it come from? Like the great matter-antimatter annihilation, it probably came soon after the universe was cool enough to have discreet charges and masses. The debris of the antimatter annihilation left one in a billion left over matter. If negative mass existed it would chase matter outward toward the observable horizon. Negative mass has both negative inertial mass and gravitational mass.
Negative mass is accelerated in the direction of positive mass, and positive mass is accelerated in the direction to be far away from negative mass.
If the absolute value of positive mass is bigger than that of negative mass, they will meet within finite time(attractive effect). If the absolute value of positive mass is smaller than that of negative mass, the distance between them will be bigger, and they cannot meet (repulsive effect).
This property is very important. Negative masses are gravitational bounded to massive positive masses (Galaxy or cluster of galaxies) for massive positive mass has attractive effect on negative mass.
One of problems regarding movement of negative mass, which researchers are likely to misunderstand is that if there are negative mass particles around the earth (or galaxy), large positive mass, they may not fly to the universe, but fall to the earth (or galaxy).
Now what am I getting at? IF negative mass is at the edges of the universe the earliest intelligent life could use it to power spacecraft in the the exotic flights we have observed on Earth regarding UFO's. Negative mass has the advantage of providing free energy.