President Donald Trump has reportedly called the Russia scandal a “cloud” hanging over his presidency. And throughout his first year in office, the weather has been very cloudy indeed.

Again and again, news about the investigations into the Trump team’s ties to Russia dominated headlines this year. And with two Trump campaign advisers indicted and set to face trial, two others flipping to become cooperating witnesses, and the president himself under scrutiny for potential obstruction of justice, there will surely be more to come next year as well.

Yet as 2017 winds down, there is still no clear answer to the central question at the heart of the probe: Did Trump’s team collude with the Russian government during the 2016 campaign?

Several Trump advisers have now acknowledged having contacts with people tied to the Russian government during the campaign. And, we now know, at least two of those conversations involved discussion of “dirt” the Russian government claimed to have on Hillary Clinton.

But we still don’t yet know whether these actually led to anything — actual coordination, say, or a secret deal of some kind between Trump’s team and the Russians.

That question remains a mystery — one that special counsel Robert Mueller’s team is now working to answer, one way or the other.

What people mean when they talk about “collusion”

....Also, there is NO COLLUSION! — Donald J. Trump (@realDonaldTrump) October 30, 2017

It was clear during the campaign that Trump had idiosyncratically pro-Russian policy positions. (He tended to criticize NATO and to praise Putin personally as a strong leader.)

And late last year, the US intelligence community concluded that the Russian government tried to interfere with the campaign to help Trump get elected. “Putin and the Russian Government developed a clear preference for President-elect Trump,” their report, released in January, reads.

The Russian effort entailed the hacking and leaks of prominent Democrats’ emails, as well as spreading pro-Trump, and anti-Clinton, messages and fake news articles on social media. (Trump himself has alternated between begrudgingly accepting the assessment that Russia was responsible and disputing it.)

The question at the heart of the scandal now is whether the Trump campaign in some way worked in concert with Moscow on these efforts.

The catchall term that has ended up being used to describe this is “collusion,” which is rather broad and vague. It also, notably, isn’t a criminal term — some forms of so-called collusion could well be legal.

But in trying to make sense of the various theories about just what that collusion might entail, I find it useful to split what investigators are reportedly looking at into two categories.

First, there’s the question of whether Trump’s team secretly worked with the Russians to interfere with the campaign somehow. If this happened and it involved Democrats’ hacked emails, there could be serious legal risk for whoever was involved, since the hacking itself was illegal. Investigators are also reportedly looking at whether the Trump team helped the Russians figure out which voters to target with their propaganda efforts.

Second, there are the darker possibilities of the sort alleged in the salacious and mostly uncorroborated Steele dossier. These include secret exchanges of money or valuable information, the Russians blackmailing or having “kompromat” on Trump, or policy promises offered by Trump’s team to Russia specifically in exchange for their assistance during the campaign (such as, perhaps, a promise to lift the sanctions President Barack Obama imposed on Russia in 2014). Obviously, any of this would be enormously scandalous if true.

What we learned about potential Trump-Russia collusion in 2017

For a while, there was very little hard evidence to support the idea that the Trump campaign could have secretly worked with the Russian government to try to swing the presidential race.

But back in March, then-FBI Director James Comey confirmed in congressional testimony that the bureau was indeed investigating “the nature of any links between individuals associated with the Trump campaign and the Russian government and whether there was any coordination between the campaign and Russia’s efforts.”

And then in July, we learned that during the campaign, Donald Trump Jr. was offered incriminating information on Hillary Clinton that a contact of his openly described as “part of Russia and its government’s support for Mr. Trump” — and that Trump Jr. emailed back, “If it’s what you say I love it.”

He proceeded to set up a meeting in Trump Tower with Jared Kushner, Paul Manafort, a Russian lawyer with ties to the Kremlin, a Russian-American lobbyist with links to Russian intelligence, and a Russian-American businessman who was once investigated for money laundering. (When news of all this broke in July, Trump Jr.’s story about what happened shifted three times in a matter of days.)

And then in late October, we learned that Trump campaign foreign policy adviser George Papadopoulos had gotten a tip that the Russian government had “dirt” in Hillary Clinton in the form of “thousands of emails.” (The revelation came in a court document that revealed Papadopoulos had pleaded guilty to making false statements to the FBI and had begun cooperating with Mueller’s team.)

Those are this year’s revelations that relate most directly to the Trump team and Russian interference with the election.

Getting further afield, we also learned, among other things, that then-National Security Adviser Michael Flynn had given a false story about his calls with Russian Ambassador Sergey Kislyak during the transition, that Kushner had tried to set up a secret communications channel with Kislyak, that Jeff Sessions failed to disclose a meeting with Kislyak, that Manafort had discussed offering private briefings to a Russian billionaire while working on Trump’s campaign, and that Trump’s company explored building a Trump Tower in Moscow during the campaign.

And of course there’s Trump’s own apparent attempts to obstruct the investigation — by pressuring then-FBI Director Comey to end the Flynn probe and then firing Comey when he refused.

In other words, there’s whole a lot of smoke. And Mueller is looking to see whether there’s fire.

The Trump team’s defense: these were isolated incidents that resulted in nothing of substance

Still, from what we know so far, it is at least possible to construe the Trump team’s various contacts with Russia as shady-looking but limited and inconclusive — because it isn’t clear what they actually led to.

For instance, all parties involved in the infamous Trump Tower meeting are publicly claiming that it was a brief and inconsequential one-off. Trump Jr. has stated that in the meeting, the Russian lawyer proved to have no useful information and quickly changed the subject to discuss US-Russia adoption policy. He’s also said there was no follow-up afterward. So far, no evidence has yet emerged that disproves this.

Meanwhile, Trump’s team has claimed that Papadopoulos was a low-level adviser with little influence on the campaign. And while Papadopoulos now admits to getting a tip that the Russians had email-related “dirt” on Clinton, we’ve heard nothing about whether he did anything with that information — say, whether he passed it on to other Trump advisers.

Trump Jr.’s DMs with WikiLeaks, meanwhile, could even be construed as exculpatory. One of them makes him seem clueless about WikiLeaks’ planned dump of John Podesta’s emails four days before it began. (“What’s behind this Wednesday leak I keep reading about?” he asked.)

Indeed, if this is in fact the sum total of what happened regarding collusion — one meeting, one tip, and a few DMs, none of which have been shown as consequential — it’s something that would look sleazy but ineffective, and not all that much like a consequential plot to swing the election.

But what else is there?

And yet it’s important to keep in mind that we didn’t know any of the above information before this year.

In 2017, what we learned essentially advanced the idea that the Trump campaign could have colluded with Russia from groundless speculation to something that’s far more plausible. (After all, the Trump Jr. emails alone are enough to make clear that people in the campaign were willing to collude with Russia.)

None of the charges Mueller’s team has filed so far are specifically about collusion, and their court documents give little indication of what they’ve found on that topic. (Flynn and Papadopoulos were charged for lying to the FBI about their contacts with Russians, while Manafort and Rick Gates were charged for financial and false statements offenses that had nothing to do with the campaign.)

And again, it is not entirely clear whether, if there was more substantial collusion between Trump’s campaigns and Russians, it would even be illegal. Coordination on something like the hacked emails could lead to criminal charges, since the hacking itself was a crime. It’s much less clear whether coordination over, say, targeting voters to spreading fake news would be illegal.

That means it’s possible that, like many scandal investigations of the past, the Mueller probe will in the end be more concerned with other, easier-to-prove crimes, like perjury, making false statements, or obstruction of justice. A focus on matters like that could prove enormously consequential for Trump’s presidency, but it wouldn’t necessarily clear up what exactly happened in 2016.

Then again, Mueller’s team has spent months subpoenaing documents, reviewing intelligence, and deposing Trump campaign and White House staffers. And Flynn and Papadopoulos are cooperating and giving investigators yet more information.

We don’t yet know where all this will lead. Perhaps 2018 will provide some sort of answer on the collusion question. This could happen through new information coming to light, or perhaps even through Mueller’s team stating that they haven’t found enough evidence on the matter to justify charges.

Or perhaps we’ll never get a clear answer. And if we don’t, the cloud Trump spoke of will never be entirely dispelled.