  Galaxy clusters are the most massive objects in the Universe and comprise a
high-temperature intracluster medium of about 10^7 K, believed to offer a main
foreground effect for cosmic microwave background (CMB) data in the form of the
thermal Sunyaev-Zel'dovich (SZ) effect. This assumption has been confirmed by
SZ signal detection in hundreds of clusters but, in comparison with the huge
numbers of clusters within optically selected samples from Sloan Digital Sky
Survey (SDSS) data, this only accounts for a few per cent of clusters. Here we
introduce a model-independent new method to confirm the assumption that most
galaxy clusters can offer the thermal SZ signal as their main foreground
effect. For the Wilkinson Microwave Anisotropy Probe (WMAP) seven-year data
(and a given galaxy cluster sample), we introduced a parameter d1 as the
nearest-neighbour cluster angular distance of each pixel, then we classified
data pixels as 'to be' (d1--> 0 case) or 'not to be' (d1 large enough) affected
by the sample clusters. By comparing the statistical results of these two kinds
of pixels, we can see how the sample clusters affect the CMB data directly. We
find that the Planck Early Sunyaev-Zel'dovich (ESZ) sample and X-ray samples
(~10^2 clusters) can lead to obvious temperature depression in the WMAP
seven-year data, which confirms the SZ effect prediction. However, each
optically selected sample (>10^4 clusters) shows an opposite result: the mean
temperature rises to about 10uK. This unexpected qualitative scenario implies
that the main foreground effect of most clusters is $not$ always the expected
SZ effect. This may be the reason why the SZ signal detection result is lower
than expected from the model.
