  Halo-based models have been successful in predicting the clustering of
matter. However, the validity of the postulate that the clustering is fully
determined by matter inside haloes remains largely untested, and it is not
clear a priori whether non-virialised matter might contribute significantly to
the non-linear clustering signal. Here, we investigate the contribution of
haloes to the matter power spectrum as a function of both scale and halo mass
by combining a set of cosmological N-body simulations to calculate the
contributions of different spherical overdensity regions, Friends-of-Friends
(FoF) groups and matter outside haloes to the power spectrum. We find that
matter inside spherical overdensity regions of size R200,mean cannot account
for all power for 1<k<100 h/Mpc, regardless of the minimum halo mass. At most,
it accounts for 95% of the power (k>20 h/Mpc). For 2<k<10 h/Mpc, haloes with
mass M200,mean<10^11 Msun/h contribute negligibly to the power spectrum, and
our results appear to be converged with decreasing halo mass. When haloes are
taken to be regions of size R200,crit, the amount of power unaccounted for is
larger on all scales. Accounting also for matter inside FoF groups but outside
R200,mean increases the contribution of halo matter on most scales probed here
by 5-15%. Matter inside FoF groups with M200,mean>10^9 Msun/h accounts for
essentially all power for 3<k<100 h/Mpc. We therefore expect halo models that
ignore the contribution of matter outside R200,mean to overestimate the
contribution of haloes of any mass to the power on small scales (k>1 h/Mpc).
