Bible says the world was formed in 7 days. But Science says it took billions of years. How do you explain this?

Technically God created the world in six days as per the Bible (Genesis 1:31) and “he rested on the seventh day from all the work that he had done” (Genesis 2:2). Coming into the question while reading the Bible one should always keep in mind that the Bible is a literary work and just like there are different genres in literary works (poetry, drama, prose, fiction and nonfiction), the Bible too has its share of genres. No one would read  A Song of Ice and Fire by George R. R. Martin (on which the famous TV series Game of Thrones is based on ) and claim that it depicts historical reality neither would anyone think that Jesus was establishing a scientific fact when he said that the mustard seed is the smallest of all seeds (Matthew 13:32) as we know that he was telling a parable and was using imageries which the Jewish people of his time could easily understand. 

Having said this the Catholic Church, though not as an infallible teaching, views the “days” of the creation story in Genesis as being symbolic and says the following:

God himself created the visible world in all its richness, diversity and order. Scripture presents the work of the Creator symbolically as a succession of six days of divine “work”, concluded by the “rest” of the seventh day. On the subject of creation, the sacred text teaches the truths revealed by God for our salvation, permitting us to “recognize the inner nature, the value and the ordering of the whole of creation to the praise of God.” – CCC 337

Being a symbolic representation of the creation event, the days mentioned in Genesis need not be taken as literal 24 hour days. Even during ancient times, church fathers like Origen and Augustine looking at the depiction of the sun being created only on the fourth day have suggested that these are not ordinary, literal days. This, that the creation account is symbolic in nature, is the “official” interpretation of the Catholic Church on the six days. Let me also make it clear that this by no means says that those who hold a literal view of the creation event as depicted in Genesis are committing a sin of dissent against the Catholic Church. 

In the Beginning is a wonderful book by then Cardinal Joseph Aloisius Ratzinger (Pope Benedict XVI) which delves into the creation narrative presented in Genesis.