The homology of $\Omega$ is $H_0(\Omega,\Bbb{Q})=H_2(\Omega,\Bbb{Q})=\Bbb{Q}$ and $H_i(\Omega,\Bbb{Q})=0$ if $i\
e 0,2$.
The Lefschetz formula tells you that a map f without fixpoints necessarily $0=Tr(H_0(f))+Tr(H_2(f)=H_0(f)+H_2(f)$. But $H_0(f)=H_0(id)=1$ for all $f$, hence in our case $H_2(f)=-1=H_2(-id)$. By the rational Hurewicz theorem and the naturality of the Hurewicz map this implies that $\pi_2(f)=\pi_2(-id)$, hence $f$ is homotopic to $-id$.