Another good question, and a point to mention that the Catholic Church has not defined everything. Some things have not been revealed to us by God, only the basic concepts are known to us. That God exists, is omnicient, eternal, and all good we accept but really cannot fully understand. That the Trinity is a reality we accept, but to fully understand in this lifetime we will not be able to do.
When we rise from the grave, we will have (if we go to heaven) a glorified body, as did Jesus after the Resurrection. He was able to pass through the walls and doors of the upper room, yet as proof that He was not just a ghost, or spirit of a dead person, St. Thomas was able to feel the holes in His hands and side. As we too will have glorified bodies like Jesus one would think that heaven would be a physical place. However the Scriptures sometimes are written in allegorical terms in order to make concepts understandable. As in Eternity there is no such thing as time, it is hard for us to comprehend Eternity, So too we are used to some sort of rule, so Heaven is described as a kingdom and the Throne of God is said to be there. God being a pure spirit, and at least until the Incarnation of Jesus, the second person of the Trinity, had no physical form. Once again as above, with a glorified body Jesus no longer was bound to the physical limitations, and need for food, space, or anything we require. There are many things that the Catholic Church has not defined, because we really don’t know. That heaven and hell exist and are eternal (will not end) and that purgatory exists as a place of final purification before we see God face to face, is something one must believe to be a Catholic. How they exist, ie are they physical places or simply states of being is subject to dispute, and not as essential as believing that they exist.
Another example of this lack of firm definition by the Catholic Church is how God created mankind. The Bible gives two stories of Creation in Genesis, both taking place over seven days, although in different orders of events. Charles Darwin advanced the theory of Evolution, and has some fairly solid evidence of Evolution. The Catholic Church does not define Science, and allows for either postion to be held by individuals, the only restriction is that a Catholic who believes in some sort of Evolution, must assent, or agree that God could have created the universe in seven days if He had willed it, and that if Evolution was the manner that God chose, at some point he created and infused an immortal and rational soul into the first humans, (which the Bible names Adam and Eve) that they where given Free will, and that is what differs us from the animals.
(as an aside, you will note St. Thomas did not according to the Scriptures put his hand in the holes in Jesus feet. In the middle east, just as today, there was a repugnance for the feet of others, as walking barefoot or in sandals one stepped in all sorts of unsavory things. Makes one think about the example of Jesus, God himself washed the feet of the Apostles, select chosen students on the night of the Last Supper. It was customary to give someone water to wash their hands, feet and face, even in the Middle East where water was precious. However to actually wash the feet of someone as a servant or slave would do. What an example for all of us.)