In terms of how his father's absence left an impact, President Obama makes some very compelling points in his memoir. The absence of his father forces him, to a great extent, to make peace with his past and his understanding of his father. The journey to Africa in the work sets the stage for an emotional reconnection with both his father as well as his own background. It does not seem to be that this is a simple ending where he understands and moves on as much as it is a complex situation where peace has to be made in order for progress to be evident. This sets up the larger truth from the President's work: Individuals cannot control what has preceded them but can control their reaction to it. The absence of his father helps the President understand who he is, what he must do, and where his father's dreams end and where his must begin.
From reading Dreams From My Father by Barack Obama, it's clear that the absence of his father left a vacuum in his life with all the unresolved attachment/separation issues that that usually entails. Every one of us can identify with that - if it is not a father we have lost then we have experienced some sort of loss, perhaps a separation, a bereavement whether of culture, identity or family member. Of course, to lose something so central as a father figure at such a young age has a profound impact. Obama seems to feel that he derived some recompense and satisfaction from the rewarding mission of bonding together with others and helping them. Identifying with black culture appeared to help him 'fill in some of the gaps' in his identity and he seems grateful for that, wanting to give something back to the people whose color and heritage he shared.