What do call individuals who express their opinions as if they were facts?

We all know some individuals who don’t express their opinions as:

I think this is going to happen...

Instead, they express it as if it were fact or news, e.g.:

Next month the price of (something) is going to increase by 10 percent.

What do you call such people? And are there any idioms or sayings to describe them?

Solution 1:

Your question title doesn't match the question body, in my opinion.

A word for someone who regularly expresses opinions as facts is pontificator:

[One who] expresses opinions or judgements in a dogmatic way.

An example would be a programmer who insists that XML is the solution to everything.

However, your example involves a prediction. It is generally understood that predictions are inherently unreliable. Yet, there seems to be a demand for such predictions anyway. Broadcast media satisfy that demand by hiring pundits:

A source of opinion; a critic

Solution 2:

Opinionated

The meaning given by OED is what we require for this case:

Conceitedly assertive and dogmatic in one’s opinions: an arrogant and opinionated man

Merriam-Webster gives a similar definition.

These definition are in slight contrast to many other dictionaries (for example American Heritage Dictionary), which say

Holding stubbornly and often unreasonably to one's own opinions.

Other less polite terms we might employ are

• Big-mouth

• Loud-mouth

Solution 3:

The term I'd use is rumour-monger (BrE) or rumormonger (AmE):

NOUN

derogatory

A person who spreads rumours.

EXAMPLE SENTENCES

A list of six names was compiled by the gossips and rumour-mongers of Belgravia, among them key figures from high society - aristocracy, government ministers and film stars.

The rumour-mongers have portrayed me as a hard-bitten political adventuress devoid of all human feeling.

What will these crazy rumour-mongers think of next?

(Definition and examples from Oxforddictionaries.com)

Solution 4:

I believe the word you're looking for is dogmatic. It means

Expressing personal beliefs and opinions as if they are certainly correct and cannot be doubted or are facts.