Depending on the measurement arrangement, every powder diffraction pattern generally also contains more or less contributions from statistical noise which causes the base-line of the diffraction pattern to deviate from the ideal flat line at zero intensity. This "background" has to be subtracted in order to be able to determine the intensities of the peaks during the peak searching. Match! uses the method of E.J. Sonneveld and J.W. Visser (described in J. Appl. Cryst. 8, 1 (1975)) to perform this task.
In order to subtract the background, either select the "Subtract background" command from the "Pattern" menu or press the corresponding button in the main toolbar at the top.