People can know the reality of love, but science cannot prove love. People can know the reality of God, but not through scientific research.
But what can be known about God--His eternal power and deity--can be understood by everyone because God has revealed it within them (see Romans 1:18-20, 2:14-15). In other words, God has given mankind the ability to learn about Him from His creation, and to some He has given a special revelation of Himself through apostles, prophets, and Jesus Christ Himself.
We can deduce clearly from all the created things that there has to be a Creator. Someone said that the chance of man's being an accident is about as reasonable as walking into a scrap-iron yard, finding a Boeing 747 jetliner, and saying, "Look how those pieces of iron flew accidentally together and formed that airplane." We are very, very complicated. For example, the neurons and nerve paths from each human eye to the human brain number some five hundred thousand. There is just no way that could happen by accident.
As we see the sunsets, the regularity of the seasons, the laws of nature, we are drawn to the fact that there has to be an intelligence behind all of it. The Bible goes on to say that people suppress the truth, because their deeds are evil (see Romans 1:18-21). They do not want to believe what is clearly shown to them.
God also reveals Himself through special revelation: The Bible. Prophets of God who have walked with Him have had special revelations. They have written these down over many years to form the book we call the Bible.
Finally, the supreme revelation of God is Jesus Christ Himself. Jesus was God come to earth. He came in fulfillment of two thousand years of Jewish history, and His coming was precisely as foretold by the prophets. He came among us and showed us what God is like, so we could know Him better. As He told His disciple Philip, "He who has seen Me has seen the Father" (John 14:9).
To sum things up, we can know God from the general revelation of creation, and we can know Him from the special revelation of those who have known Him--and especially from the life and words of Jesus Himself.
People who say there is no God must realize that atheism takes a great deal more faith than does belief in God. Faith in God simply makes more sense! When you consider scientific theories regarding the beginning of the cosmos, you are struck with the fact that there have been at least ten major "cosmogonies" during the last two hundred years. Man is continuously changing his theory of how it all came to be. As our knowledge expands, we shift and shift and shift. But so far, no one has ever come up with anything better than the biblical account that there is a creator God who, in the beginning, made all that is.