Steigman's doctoral dissertation showed that the hypothesis of cosmological matter–antimatter symmetry is false and in our universe there be must be a significant excess of baryons over antibaryons.
Beginning in 1972 he spent 23 summers at the Aspen Center for Physics, where he served as a trustee in 1978–83, a member of the advisory board in 1983–98, and a longtime organizer of astrophysics workshops.
[2] Steigman and Schramm received first prize in the 1980 Gravity Research Foundation essay competition for their paper which analyzed a hypothetical universe dominated by massive neutrinos.
With David Schramm, Michael Turner, Keith Olive, and Terrence P. Walker, Gary Steigman derived accurate estimates of the baryon density of the universe and constrained particle properties.
Around the same time, he began a romantic, interhemispheric relationship with a Brazilian astronomer, Sueli Viegas, from the Institute of Astronomy, Geophysics and Atmospheric Sciences at the University of São Paulo, whom he had met at a conference in Rio de Janeiro.