I think that this is a great question because it touches on something that we observe frequently in our lives but few people bother asking "why does this happen?" In order to answer your question I am going to have to assume that you know a little bit of chemistry, but hopefully the answer will be understandable even if you haven't had a high school chemistry class.

You may know that all materials are made from atoms, the basic building block of matter that themselves consist of three main components: neutrons, protons, and electrons. The neutrons and protons sit in the nucleus of the atom while the electrons whiz around and orbit the nucleus, much like the planets orbit the sun. The electrons can be positioned relatively closely to the nucleus, like Mercury is positioned closely to the sun, or be positioned relatively far from the nucleus, like Neptune, depending on their energy (picture energy in the following way: a car moving 100 miles per hour has much more energy than a car moving 20 miles per hour). As the electrons acquire more energy their orbit tends to become positioned further and further from the nucleus. A critical concept to understand, however, is that electrons cannot be positioned just anywhere; instead, complicated science tells us that electrons can only be positioned certain distances away from the nucleus. The analogy to the solar system would be that a planet could be positioned where Mercury, Venus, Earth, etc. are but nowhere in between!

Now what happens when we shine light onto a material? There are two possibilities: it can either get absorbed by the material or pass right through. You may have learned during your studies on light energy that each frequency of light has a different energy (red light has the lowest energy and violet the highest energy). If the light that you are shining onto the material has at least the energy required to "bump" an electron to a position further from the nucleus (i.e. move it into a higher energy state), the light will be absorbed! If the light doesn't have the requisite energy it will pass straight through, just as it does in glass.

In aluminum, and indeed in all metals, the electrons are very "loosely held" to the nucleus and the energy required to "bump" and electron into a position further from the nucleus is very low compared to the energy of visible light. Thus metals tend to absorb visible light very well, which is why they appear opaque.

Bonus: But if metals absorb all visible light, how can we see them? After absorption occurs two things may occur: either the energy is used from some arbitrary purpose or light is re-emitted from the material. In metals, a good portion of the absorbed light is re-emitted and this re-emitted light is actually what we see! An interesting fact here is that metals like silver and aluminum, which appear gray, absorb and re-emit all the different colors of light equally while metals like gold and copper, which appear reddish, absorb the higher energy frequencies (like blue) more efficiently and tend to not re-emit blue light.