There is no concrete proof of God's existence. Even the Bible says, "And without faith it is impossible to please God, because anyone who comes to Him must believe that He exists and that He rewards those who earnestly seek Him" (Hebrews 11:6). Of course He could prove His existence in a moment if He so desired, but He wants us to come to Him because we want to by faith, not because we can no longer deny His existence.

He has proven His existence in MY life, by answering prayer and performing miracles. And for me, further proof of His existence lies in the minute details that add vitality to all life -- for example, how fragile, yet so perfectly complex, our minds and nervous systems are; or how every little thing serves some purpose in keeping our atmospheres so balanced.

If you're looking for the reason for suffering, why some starve and why others do not, if you're looking for the perfect world -- well, God gave us one and we ruined it by sinning against Him. And even before that, Lucifer created the possibility for sin by attempting to overthrow God. God created us all with freewill -- so why does He need to come down and fix all of our problems on a whim when He never created them, and when we have the means to do it ourselves, but apparently lack the willpower?