Weak lensing surveys based on the mass aperture found a substantial
number of detections without any visible counterpart, the so--called
dark halos, i.e. dark matter halos which apparently fail to produce
any kind of radiation. Some authors argue that dark haloes are
physical objects not predicted by the standard scenario of structure
formation. We analiyzed the Garching-Bonn-Deep-Survey (GaBoDS) with a
new optimized filter and fixing the detection threshold accordingly to
the noise properties of the data.  This has been done thanks to a
statistical analysis of the data sample. We also realized numerical
simulations to better understand the role of the large scale structure
contamination in weak lensing surveys and to test our new filter.
Finally we briefly summarize the observational opportunities that weak
lensing by galaxy clusters on the Cosmic Microwave Background may open
thanks to the upcoming millimetric observatories, like ACT, SPT and
ALMA.