Check the paper Antonelli, P. L.; Burghelea, D.; Kahn, P. J. The non-finite homotopy type of some diffeomorphism groups.The non-finite homotopy type of some diffeomorphism groups. Topology 11 (1972), 1–49.
There they mention an old result of Palais (Homotopy theory of infinite dimensional manifoldsHomotopy theory of infinite dimensional manifolds. Topology 5 (1966), 1-16) which states that the identity component of ${\rm Diff}_0(M)$ has the homotopy type of a countable $CW$-complex.
In their paper, Antonelli, Burghelea and Kahn prove that for many smooth manifolds (including spheres of dimension $\geq 7$) the group ${\rm Diff}_0(M)$ does not have the homotopy type of a finite $CW$-complex. (This is highly nontrivial.)
Above, by diffeomorphisms they mean smooth diffeomorphisms and the topology is the $C^\infty$-topology.