The universe is vast. The Earth we live on appears to be just a speck of dust circling a small star in a remote corner of an obscure galaxy. If we are a speck in the immense space, we also occupy dnly an instant in the expanse of time. The universe is also very old. We now know that it is about fifteen or twenty billion years OM, while we, the human beings have been around for only two million years or so. This vast and ancient universe is populated with a variety of objects. Let us now understand what objects constitute the universe and how they are distributed. In other words, what the physical structure of the universe is. In doing so, we will not go into its chemical composition or aoy other details. However, before we embark on this venture, we will give you an idea of the cosmic distances, so that you're able to appreciate what follows.