I was reading an article claiming that the Israelites practised Yahwism originally. After the Babylonian exile, they thought Yahweh was angry with them and decided to appease Yahweh by denouncing the other gods and rewriting the books to be monotheistic, giving us the Hebrew bible as we know it.
I was wondering if this theory was based on any evidence, thanks.
Yes, this is an entirely mainstream viewpoint, though the word 'rewritten' is misleading. Don't think of the Hebrew Bible as a revised version of an older polytheistic version. Think of it as containing traces of older henotheistic and polytheistic traditions.
Quick note on terminology:
What we see in the Hebrew Bible in its present form is monotheism, with plenty of traces of pre-exilic henotheism -- an acceptance that different places cultivate different gods, and Yahweh is the one that Israel cultivates.
Bear in mind that the Hebrew Bible isn't a single text subject to a single phase of composition and editing, it's a compilation of many texts. You can find traces of henotheism in older material like Deuteronomy-Joshua-Judges and Micah; not so much in later texts like Chronicles or Daniel.
Some examples. Psalm 82 casts Elohim (an alternate name for Yahweh) as a member of the divine council of the Canaanite god El. Here's an adjusted form of the NRSV translation: the NRSV translates some names into conventional English expressions; here, names in italics are given as they appear in the Hebrew.
Elohim has taken his place in the council of El;
in the midst of the gods he holds judgement:
...
'I say, "You are gods,
children of El, all of you;
nevertheless, you shall die like mortals,
and fall like any prince ..."'
Compare Deuteronomy 32.8-12 (again, with Hebrew names in italics, in place of the NRSV translations):
When Elyon apportioned the nations,
when he divided humankind,
he fixed the boundaries of the peoples
according to the number of the gods;
Yahweh's own portion was his people,
Jacob his allotted share.
...
Yahweh alone guided [Jacob];
no foreign god was with him.
Various other passages give indications of henotheism: Exodus 12.12 + Numbers 33.4; Deuteronomy 32.43; Judges 11.24; Micah 4.5. 2 Kings 17 describes immigrants in Samaria being accepted in spite of their foreign customs in practising polytheism. Other passages indicate transitions from one henotheism to the henotheism of Yahweh, or from polytheism to henotheism, or from henotheism to polytheism: Joshua 24; 1 Kings 11.1-8. Note also the wording of the first commandment: it isn't 'Don't believe in the existence of other gods', it's 'Don't worship other gods'.
Aside from religious practice, we find celestial phenomena acting as symbols of polytheism in a couple of important passages -- Deuteronomy 4.19:
And when you look up to the heavens and see the sun, the moon, and the stars, all the host of heaven, do not be led astray and bow down to them and serve them, things that Yahweh your God has allotted to all the peoples everywhere under heaven.
With this you can compare the first creation narrative in Genesis 1, especially 3-8 and 14-19, which heavily emphasise that it's Elohim (= Yahweh) who created celestial phenomena, and not that they belong to other divinities. That is, the first creation narrative is in part a polemic against polytheism, more monotheistic than the other passages I've mentioned.
As I said at the start, this is absolutely the standard mainstream position among biblical scholars: that there was a shift in the pre-exilic period from a henotheistic cult of Yahweh as a national god, towards a monotheistic cult teaching that Yahweh is the only god, which solidified into a harder monotheism as time went on. There's a lot of disagreement among biblical scholars as well, of course, for example over the place of Asherah in Yahweh-worship, or the nature of the relationship between Yahweh and the Canaanite pantheon. For further reading you can try --