The nature of matter and our senses is such that we can detect only a millionth part of the wonders around us. Can we see the wind?  Can our ears detect the sound of a dog whistle?  Our vision and hearing is confined to that narrow part of the electromagnetic spectrum that we identify as visible light and sound.  An angel may be sitting on your shoulder, and you may never know it unless you open your heart to the heavenly messenger and receive the message that God has for you.