Learn Portuguese (Portugal): De acordo com eles, o óvni estaria no céu. - According to them, the UFO was allegedly in the sky.

This sentence uses the evidential conditional, often called the conditional of rumor. By using estaria instead of a standard past tense, the speaker signals that they are merely reporting a claim they heard, distancing themselves from its factual accuracy. This is why it translates to 'was allegedly' or 'was reportedly' in English.

Does using the conditional here mean the speaker thinks the UFO sighting is a lie?

Not necessarily a lie, but it explicitly removes the speaker's endorsement of the claim. It acts as a safeguard, which is very common in Portuguese journalism. If the speaker completely verified it as a fact, they would just say estava. Using estaria shifts all responsibility for the truth onto 'them' (eles).

Is óvni treated like a regular noun, and what does it stand for?

Yes, it is an acronym that became a standard noun, standing for Objeto Voador Não Identificado (Unidentified Flying Object). Because it is pronounced as a single word rather than spelled out letter-by-letter, it requires the acute accent on the ó to keep the stress on the first syllable, matching standard Portuguese spelling rules.

Could I start the sentence with Segundo eles instead of De acordo com eles?

Absolutely. Segundo eles is a perfect synonym and is often preferred in spoken Portuguese because it is shorter. It flows very naturally when reporting rumors or another person's perspective.