Could much of the “missing” antimatter make up neutrons?

A neutron can decay into a proton, an electron, and neutrino. Could an antiproton, a positron, and a neutrino combine into a neutron? Could this be where much of the "missing" antimatter is?

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well, this is completely non sense as far as quantum numbers go.neutron-->proton electron anti_e_neutrino. antineutron--> antiproton positron e_neutrino. –  anna v Jan 3 '13 at 4:45
The neutron is not its own antiparticle. The antineutron is a distinct particle. It happens to have the same charge as the neutron (zero), but it has other characteristics that distinguish it. –  Keith Thompson Jan 3 '13 at 6:33

Assuming that by "negatron" you mean a anti-proton, then no. It violated the conservation of baryon number (for aproton $B = -1$ and for a neutron $B = 1$) and in any case, free neutrons are unstable--it's not just that they can decay, they will decay.