  Recently, a few-degrees extended $\gamma$-ray halo in the direction of
Geminga pulsar has been detected by HAWC, Milagro and Fermi-LAT. These
observations can be interpreted with positrons ($e^+$) and electrons ($e^-$)
accelerated by Geminga pulsar wind nebula (PWN), released in a Galactic
environment with a low diffusion coefficient ($D_0$), and inverse Compton
scattering (ICS) with the interstellar radiation fields. We inspect here how
the morphology of the ICS $\gamma$-ray flux depends on the energy, the pulsar
age and distance, and the strength and extension of the low-diffusion bubble.
In particular we show that $\gamma$-ray experiments with a peak of sensitivity
at TeV energies are the most promising ones to detect ICS halos. We perform a
study of the sensitivity of HAWC, HESS and the future CTA experiment finding
that, with efficiencies of the order of a few %, the first two experiments
should have already detected a few tens of ICS halos while the latter will
increase the number of detections by a factor of 4. We then consider a sample
of sources associated to PWNe and detected in the HESS Galactic plane survey
and in the second HAWC catalog. We use the information available in these
catalogs for the $\gamma$-ray spatial morphology and flux of these sources to
inspect the value of $D_0$ around them and the $e^{\pm}$ injection spectrum.
All sources are detected as extended with a $\gamma$-ray emission extended
about $15-80$ pc. Assuming that most of the $e^{\pm}$ accelerated by these
sources have been released in the interstellar medium, the diffusion
coefficient is $2-30 \cdot 10^{26}$ cm$^2$/s at 1 TeV, i.e. two orders of
magnitude smaller than the value considered to be the average in the Galaxy.
These observations imply that Galactic PWNe have low-diffusion bubbles with a
size of at least 80 pc.
