If the result of combining matter with anti-matter is ONLY positive energy then you have created something out of nothing.

Incidentally weight is the effect of gravity on mass.

Mass is a measure of the inertia of an object.

An object with a mass of 1 ton would have a weight of 1 ton on the surface of the Earth but a much lower weight on the surface of the moon.

However the force required to accelerate the mass of 1 ton to 1mph would be the same whether it be on the Earth, the Moon or in space.