At biblegateway.com you will find, in the New International Version, the following explanation under footnote "c" which is annotated right on the blank verse at 37:
Some manuscripts include here Philip said, “If you believe with all your heart, you may.” The eunuch answered, “I believe that Jesus Christ is the Son of God.” - Acts 8:37 New International Version
Sometimes a scripture is found in several manuscripts but not in others. This suggests that some copyist added verses to other manuscripts. I believe that scholars tend to choose the earliest manuscript as the most "authentic" one.