Additional Study Information: The purpose of this study is to evaluate if the study drug, niraparib will help patients delay or prevent their breast cancer from returning. This study will compare niraparib to placebo in study participants who have been treated for breast cancer and were then found to have breast cancer ctDNA (Circulating tumor DNA (ctDNA) is found in the bloodstream and refers to DNA that comes from cancerous cells and tumors) in their blood. Niraparib is a tablet that is taken once a day. A placebo, or sugar tablet, is a tablet that is made to look like the active drug but does not have the active ingredient in it. Researchers use a placebo to see if the study drug works better than not taking anything.