Do Buddhists believe in God or gods?

In a creator God, no - the early texts do not sidestep the question, they argue with it directly. Devas (deva, plural devā), celestial beings, do exist in Buddhist cosmology, but they remain inside saṃsāra and count as neither creators nor a source of liberation.

The argument against a creator is made concretely in the suttas. In DN 1, as a world-period unfolds again, one being is reborn first in an empty palace of Brahmā; others appear there later, and it concludes that it made them, while they take its word for it. In MN 49 the Brahmā named Baka holds his world to be eternal and beyond death, and the Buddha takes that delusion apart piece by piece. In DN 11 a monk tours the heavens asking where the four great elements cease without remainder; the Great Brahmā, in front of his retinue, first recites his titles of creator and lord, then draws the monk aside and admits he does not know. AN 3.61 rejects issaranimmānahetu, the claim that everything a person feels - pleasant, painful or neutral - is caused by the creative act of a lord; it rejects total karmic determinism and pure causelessness in the same breath.

Devas live long, wield great power and enjoy great pleasure, but their span runs out: they die and are reborn according to their kamma, sometimes lower down. Iti 83 describes the five signs by which other devas recognise that one of them is about to die, and calls a human birth a good destination for the one departing. Not all heavens are alike: the Brahmā realms are not a matter of sense pleasure at all. Brahmā in the Pali Canon names a class of high celestial beings, Baka being one of them, and is not the impersonal brahman of the Upanishads.

A Buddhist takes refuge in the Buddha, the Dhamma and the Sangha, not in a deva. Devas, meanwhile, come to the Buddha and ask him questions all through the suttas - the whole Devatā-saṃyutta opens that way (SN 1.1). Rebirth among the devas is called a good fruit of virtue (MN 41), but not the end of the path: a heaven is not nibbāna and guarantees no liberation.

“The Buddha left questions about the origin of the world unanswered” is not quite right. Of the ten undeclared questions (avyākata, MN 63, MN 72), four concern the world (whether it is eternal or not, finite or not), two the identity of soul and body, and four the existence of a Tathāgata after death. Who made the world is not among them. Cosmogonic accounts do exist: DN 27 and DN 1 describe the world contracting and expanding again, AN 4.156 distinguishes four incalculable phases of an aeon (kappa), and no starting point for saṃsāra is assigned (SN 15.1). The arising of suffering and its ending are explained through conditions (paṭiccasamuppāda) rather than through anyone’s will.

So the word “atheistic” needs a qualification. Buddhism is non-theistic with respect to a creator God: it does not deny that deities exist, it just gives them no role as creator or saviour. That does not stop it from being a religious tradition with a path to liberation (nibbāna).

All of the above concerns the Pali Canon. Many Mahayana traditions have well-developed devotional practices addressed to Amitābha, the Buddha of the Pure Land schools, and to Guanyin, the Chinese form of the bodhisattva Avalokiteśvara, usually depicted as female. Those figures and practices belong outside the Pali Canon, and they are not regarded as creators either.

The canon states the general principle plainly: “You yourselves must make the effort; the Tathāgatas only proclaim the way” (Dhp 276).

Where to read: MN 49 on the Buddha and Baka the Brahmā; DN 11 on the Brahmā with no answer; AN 3.61 on the doctrine of creation by a lord.