We study the weight 2 graded piece of the compactly supported rational cohomology of the moduli spaces of curves M g,n and show that this can be computed as the cohomology of a graph complex that is closely related to graph complexes arising in the study of embedding spaces. For n = 0, we express this cohomology in terms of W 0 H • c (M g ′ ,n ′ ) for g ′ ≤ g and n ′ ≤ 2, and thereby produce several new infinite families of nonvanishing unstable cohomology groups on M g , including the first such families in odd degrees. In particular, we show that dim H 4g−k (M g ) grows at least exponentially with g, for k ∈ {8,