Our finite minds find it hard to comprehend that before the universe was created, God existed alone. We know from John 1:1 that Jesus also existed: “In the beginning was the Word and the Word was with God, and the Word was God.” The preincarnate Christ was intimately united with the Father, so as to partake of His glory and to be appropriately called God. He has Himself explained it in John 17:5: “And now Father, glorify Me with Yourself with the glory which I had with You before the world was.”
We also know that the Holy Spirit was present before we were created. Genesis 1:2 describes the Spirit “hovering over the face” of the dark and formless earth. So, before time even existed, God existed in three Persons: Father, Son and Holy Spirit. The Trinity existed in perfect harmony and flawlessness, having all they needed in one another. David said in Psalms 16:11 that "joy and pleasures forever more" are in the presence of God. That means to be in the presence of God carries with it an overwhelming sense of joy, fulfillment, and pleasure. Before creation, God felt complete joy and fulfillment as He perfectly beheld and communed with Himself. God has and always will experience complete joy because He has complete and perfect knowledge of Himself.
So before He created the universe, God experienced absolute satisfaction in Himself. God dwelt joyfully alone in eternity as the Trinity. These three were together in fellowship with one another from all eternity. They loved each other. We know at some point they discussed the redemption of mankind (Ephesians 1:4-5; 2 Timothy 1:9; John 17:24), but everything else lies in mystery.