I want to remove a transparent dark backgrond outside of dialog box.

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In addition to chuky's answer;
If your minSdkVersion value is greater than or equal to 14, you can use setDimAmount() method.
dialog.getWindow().setDimAmount(float amount);
According to reference;
amount The new dim amount, from 0 for no dim to 1 for full dim.
or
As stated previously, you can clear window flag.