Oh man I love talking about our cube! The cube is called the Kaaba, which means ‘the cube’. It is very holy (you are meant to be visualising it as a sort of aiming point every time you pray) but it isn’t secret, and a lot of people assume it is inherently mysterious.
So many pre-Islamic Arabian cities, including Mecca, had a cube. These served as public shrines and housed idols of the pagan gods of that city. A lot of them also housed or had embedded into them meteorites, because something falling from the sky is pretty easy to worship.
Our cube, the cube, is believed to be the site where way back in the day Ibrahim (Abraham, if you want) built a house of worship to God. Over time as monotheism lost out to pagan polytheism its original purpose was lost and people use it as a shrine and copy the form of that shrine elsewhere.
For any Muslim narrative, one of the things that Muhammad does as a prophet is to kick Arabia back onto the monotheism from which it had fallen away (into ignorance, jahiliyyah). So when Muhammad re-enters Mecca he goes to the Kaaba and smashes all the idols inside with a staff while saying that ‘truth has come and falsehood has vanished’ - in some tellings of this he preserves a statue of the infant Jesus and Mary but places it outside. What happens next is a kind of religious compromise where he Islamifies (well, God does, but he’s the one telling Muhammad what to do) bits of the Meccan pagan religion. So the cube stays, the meteorite in it, a black stone, stays, and the Meccan fertility rite of tawaf where people circled the cube naked gets changed so you have to wear clothes and it isn’t horny any more. A mosque is built up around it. Eventually the direction you face during prayer is changed from towards Jerusalem to towards the Kaaba. And from then until now Islamic practice in relation to the cube has not really changed.
FUN CUBE FACTS
It’s not the same building Ibrahim built or the same building Muhammad resanctified. It has fallen down in earthquakes or been destroyed with catapults an alarming number of times and then just rebuilt in a very practical way. The current incarnation dates back to 1626.
It’s not black - in high winds or when it’s being changed, which it is yearly, you can see under the black and gold cloth covering, the kiswah, and see the granite blocks underneath:
The kiswah also doesn’t have to be black. It’s a pretty firmly established tradition now but it’s been red, green and white at various times.
It has an inside. There’s nothing secret about it like the Holy of Holies, every year a bunch of dignitaries get to go in and clean it. There’s nothing material important in there, that being sort of the point:
The little cabinet has cleaning supplies, perfumes and the like. If you pray inside the Kaaba you can face in any direction, plus I like to imagine you unlock a fun little achievement.