30 approx. silver balls creating formations in the sky before vanishing

My wife and two sons were in our backyard. From the side yard, my wife yelled for me and one of my sons to come look at what she was seeing. She was staring up into the sky. I walked up beside her and saw approximately 30 silver balls in the sky. When I first saw them, most of them were in a triangle shape with the remaining balls (around 7), hovering around the formation at varying distances. They moved out of formation and would come close to each other and then move away again. Around 10 came into a triangle formation again, but exited the formation only about 2 or 3 minutes later. After they exited the formation, they appeared to group together with some of them vanishing while others either stayed close to the same place and others flying around in no discernible pattern. They primarily maintained the same altitude. We watched them for around 15 minutes before we went inside with our sons. My wife was afraid of what they might be or what they might do. ! I came back out around 30 minutes later but they were all gone.