A broadband femtosecond supercontinuum (SC) light with a dark core has been generated in a pump-probe type geometry. It has been achieved by introducing a small thermal perturbation in a highly absorbing medium with a continuous wave pump laser. The SC was generated with the help of a photonic crystal fiber and a nJ femtosecond oscillator. Numerical simulations for the system were done by employing the Fresnel-Kirchhoff formula. The power dependence of the dark core diameter was also studied experimentally, and the results agree well with numerical simulations. The dark core size can be controlled by varying the input power of the pump beam.